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/tests/test_message_schema.rs

@@ -0,0 +1,40 @@
+mod tests {
+    use rmcp::model::{ClientJsonRpcMessage, ServerJsonRpcMessage};
+    use schemars::schema_for;
+
+    #[test]
+    fn test_client_json_rpc_message_schema() {
+        let schema = schema_for!(ClientJsonRpcMessage);
+        let schema_str = serde_json::to_string_pretty(&schema).unwrap();
+        let expected = std::fs::read_to_string(
+            "tests/test_message_schema/client_json_rpc_message_schema.json",
+        )
+        .unwrap();
+
+        // Parse both strings to JSON values for more robust comparison
+        let schema_json: serde_json::Value = serde_json::from_str(&schema_str).unwrap();
+        let expected_json: serde_json::Value = serde_json::from_str(&expected).unwrap();
+        assert_eq!(
+            schema_json, expected_json,
+            "Schema generation for ClientJsonRpcMessage should match expected output"
+        );
+    }
+
+    #[test]
+    fn test_server_json_rpc_message_schema() {
+        let schema = schema_for!(ServerJsonRpcMessage);
+        let schema_str = serde_json::to_string_pretty(&schema).unwrap();
+        let expected = std::fs::read_to_string(
+            "tests/test_message_schema/server_json_rpc_message_schema.json",
+        )
+        .unwrap();
+
+        // Parse both strings to JSON values for more robust comparison
+        let schema_json: serde_json::Value = serde_json::from_str(&schema_str).unwrap();
+        let expected_json: serde_json::Value = serde_json::from_str(&expected).unwrap();
+        assert_eq!(
+            schema_json, expected_json,
+            "Schema generation for ServerJsonRpcMessage should match expected output"
+        );
+    }
+}