feat: Add support for Tool.outputSchema and CallToolResult.structuredContent (#316)

* feat: add output_schema field to Tool struct

- Add optional output_schema field to Tool struct for defining tool
output structure
- Update Tool::new() to initialize output_schema as None

* feat: add structured_content field to CallToolResult

- Add optional structured_content field for JSON object results
- Make content field optional to support either structured or
unstructured results
- Add CallToolResult::structured() and structured_error() constructor
methods

* feat: implement validation for mutually exclusive content/structuredContent

- Add validate() method to ensure content and structured_content are
mutually exclusive
- Implement custom Deserialize to enforce validation during
deserialization
- Update documentation to clarify the mutual exclusivity requirement

* feat: add output_schema support to #[tool] macro

- Add output_schema field to ToolAttribute and ResolvedToolAttribute
structs
- Implement automatic output schema generation from return types
- Support explicit output_schema attribute for manual specification
- Generate schemas for Result<T, E> where T is not CallToolResult
- Update tool generation to include output_schema in Tool struct

* feat: implement IntoCallToolResult for structured content

- Add Structured<T> wrapper type for explicit structured content
- Implement IntoCallToolResult for Structured<T> with JSON serialization
- Add support for Result<Structured<T>, E> conversions
- Enable tools to return structured content through the trait system

* fix: update simple-chat-client example for optional content field

- Handle Option<Vec<Content>> in CallToolResult.content
- Add proper unwrapping for the optional content field
- Fix compilation error in chat.rs

* fix: update examples and tests for optional content field

- Add output_schema field to Tool initialization in sampling_stdio
example
- Update test_tool_macros tests to handle Option<Vec<Content>>
- Use as_ref() before calling first() on optional content field

* feat: implement basic schema validation in conversion logic

- Add validate_against_schema function for basic type validation
- Add note that full JSON Schema validation requires dedicated library
- Document that actual validation should happen in tool handler

* feat: add structured output support for tools

- Add output_schema field to Tool struct for defining output JSON schemas
- Add structured_content field to CallToolResult (mutually exclusive with content)
- Implement Structured<T> wrapper for type-safe structured outputs
- Update #[tool] macro to automatically generate output schemas from return types
- Add validation of structured outputs against their schemas
- Update all examples and tests for breaking change (CallToolResult.content now Option)
- Add comprehensive documentation and rustdoc
- Add structured_output example demonstrating the feature

BREAKING CHANGE: CallToolResult.content is now Option<Vec<Content>> instead of Vec<Content>

Closes #312

* fix: correct structured output doctest to use Parameters wrapper

The #[tool] macro requires Parameters<T> wrapper for tool inputs.
This fixes the pre-existing broken doctest in the structured output
documentation example.

* feat: replace Structured<T> with Json<T> for structured output

- Remove Structured<T> type definition and implementations
- Reuse existing Json<T> wrapper for structured content
- Update IntoCallToolResult implementations to use Json<T>
- Add JsonSchema implementation for Json<T> delegating to T
- Update all examples and tests to use Json<T> instead of Structured<T>
- Update documentation and exports

BREAKING CHANGE: Structured<T> has been replaced with Json<T>. Users must update their code to use Json<T> for structured tool outputs.

* feat: add output_schema() method to IntoCallToolResult trait

- Add output_schema() method with default None implementation
- Implement output_schema() for Json<T> to return cached schema
- Implement output_schema() for Result<Json<T>, E> delegating to Json<T>
- Enable trait-based schema generation for structured outputs

* feat: update macro to detect Json<T> wrapper for output schemas

- Add extract_json_inner_type() helper to detect Json<T> types
- Update schema generation to only occur for Json<T> wrapped types
- Remove generic Result<T, E> detection in favor of specific Json<T> detection
- Add comprehensive tests to verify schema generation behavior

* feat: add builder methods to Tool struct for setting schemas

- Add with_output_schema<T>() method to set output schema from type
- Add with_input_schema<T>() method to set input schema from type
- Both methods use cached_schema_for_type internally
- Add comprehensive tests for builder methods

* fix: address clippy warnings

- Add Default implementation for StructuredOutputServer
- Fix collapsible else-if in simple-chat-client
- No functional changes

* style: apply cargo fmt

Apply automatic formatting changes to:
- examples/simple-chat-client/src/chat.rs - fix line wrapping
- crates/rmcp-macros/src/tool.rs - format method chaining
- examples/servers/src/structured_output.rs - reorder imports and format function signatures

* chore: fix formatting

* chore: fix rustdoc redundant link warning

* refactor: validate_against_schema

* feat: enforce structured_content usage when output_schema is defined

This commit implements strict validation to ensure tools with output_schema
consistently use structured_content for both success and error responses.

Changes:
- Enhanced ToolRouter::call() validation to require structured_content when output_schema is present
- Added validation that tools with output_schema cannot use regular content field
- Added comprehensive tests covering the new strict validation behavior
- Created example demonstrating proper structured output usage
- Updated TODO.md to track validation improvements

This ensures consistent response format and better type safety for MCP clients.

* chore: remove TODO.md

* refactor: simplify output schema extraction logic in tool macro

- Extract complex nested logic into dedicated helper function
- Replace deeply nested if-else chains with functional approach
- Use early returns and ? operator for cleaner code flow
- Reduce 46 lines to 7 lines in main logic while improving readability

* chore: run cargo fmt

* fix: enforce structured_content usage when output_schema is defined

Structured content is returned as a JSON object in the structuredContent
field of a result.For backwards compatibility, a tool that returns
structured content SHOULD also return the serialized JSON in a
TextContent block.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

Tools may also provide an output schema for validation of structured
results. If an output schema is provided:

- Servers MUST provide structured results that conform to this schema.
- Clients SHOULD validate structured results against this schema.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#output-schema

* chore: cargo fmt

crates/rmcp/src/lib.rs

@@ -48,8 +48,52 @@
 //! }
 //! ```
 //!
-//! Next also implement [ServerHandler] for `Counter` and start the server inside
-//! `main` by calling `Counter::new().serve(...)`. See the examples directory in the repository for more information.
+//! ### Structured Output
+//!
+//! Tools can also return structured JSON data with schemas. Use the [`Json`] wrapper:
+//!
+//! ```rust
+//! # use rmcp::{tool, tool_router, handler::server::tool::{ToolRouter, Parameters}, Json};
+//! # use schemars::JsonSchema;
+//! # use serde::{Serialize, Deserialize};
+//! #
+//! #[derive(Serialize, Deserialize, JsonSchema)]
+//! struct CalculationRequest {
+//!     a: i32,
+//!     b: i32,
+//!     operation: String,
+//! }
+//!
+//! #[derive(Serialize, Deserialize, JsonSchema)]
+//! struct CalculationResult {
+//!     result: i32,
+//!     operation: String,
+//! }
+//!
+//! # #[derive(Clone)]
+//! # struct Calculator {
+//! #     tool_router: ToolRouter<Self>,
+//! # }
+//! #
+//! # #[tool_router]
+//! # impl Calculator {
+//! #[tool(name = "calculate", description = "Perform a calculation")]
+//! async fn calculate(&self, params: Parameters<CalculationRequest>) -> Result<Json<CalculationResult>, String> {
+//!     let result = match params.0.operation.as_str() {
+//!         "add" => params.0.a + params.0.b,
+//!         "multiply" => params.0.a * params.0.b,
+//!         _ => return Err("Unknown operation".to_string()),
+//!     };
+//!     
+//!     Ok(Json(CalculationResult { result, operation: params.0.operation }))
+//! }
+//! # }
+//! ```
+//!
+//! The `#[tool]` macro automatically generates an output schema from the `CalculationResult` type.
+//!
+//! Next also implement [ServerHandler] for your server type and start the server inside
+//! `main` by calling `.serve(...)`. See the examples directory in the repository for more information.
 //!
 //! ## Client
 //!
@@ -104,6 +148,9 @@ pub use handler::client::ClientHandler;
 #[cfg(feature = "server")]
 #[cfg_attr(docsrs, doc(cfg(feature = "server")))]
 pub use handler::server::ServerHandler;
+#[cfg(feature = "server")]
+#[cfg_attr(docsrs, doc(cfg(feature = "server")))]
+pub use handler::server::wrapper::Json;
 #[cfg(any(feature = "client", feature = "server"))]
 #[cfg_attr(docsrs, doc(cfg(any(feature = "client", feature = "server"))))]
 pub use service::{Peer, Service, ServiceError, ServiceExt};