feat(model): add json schema generation support for all model types (#176)

* feat(model): add json schema generation support for all model types

This commit adds JSON Schema support for all model types by implementing
`schemars::JsonSchema` trait. The feature is gated behind the new
`schemars` feature flag. This enables automatic schema generation for
API documentation and validation purposes. Added tests to verify schema
generation for client and server JSON-RPC messages.

* fix(model): add manual json schema implementation for `NumberOrString`

This commit adds a manual implementation of `JsonSchema` trait for the
`NumberOrString` enum to properly represent its union type nature in
JSON Schema. The schema now correctly specifies that the type can be
either a number or a string using the `oneOf` validation keyword.

* fix(model): skip extensions field in json schema generation

The `Extensions` type was incorrectly included in JSON schema
generation, which could lead to confusing API documentation. This commit
adds `#[schemars(skip)]` attribute to all `extensions` fields in request
and notification structs, and removes the manual `JsonSchema`
implementation for the `Extensions` type since it's an internal
implementation detail that shouldn't be exposed in the schema.

crates/rmcp/src/model/annotated.rs

@@ -10,6 +10,7 @@ use super::{
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default)]
 #[serde(rename_all = "camelCase")]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct Annotations {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub audience: Option<Vec<Role>>,
@@ -36,6 +37,7 @@ impl Annotations {
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct Annotated<T: AnnotateAble> {
     #[serde(flatten)]
     pub raw: T,