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

Author: shorwood

README.md

@@ -192,6 +192,7 @@ See [examples](examples/README.md)
 - `client`: use client side sdk
 - `server`: use server side sdk
 - `macros`: macros default
+- `schemars`: implement `JsonSchema` for all model structs
 
 ### Transports
 

crates/rmcp/Cargo.toml

@@ -28,7 +28,7 @@ paste = { version = "1", optional = true }
 oauth2 = { version = "5.0", optional = true }
 
 # for auto generate schema
-schemars = { version = "0.8", optional = true }
+schemars = { version = "0.8", optional = true, features = ["chrono"] }
 
 # for image encoding
 base64 = { version = "0.21", optional = true }
@@ -89,6 +89,7 @@ tower = ["dep:tower-service"]
 __auth = ["dep:oauth2", "dep:reqwest", "dep:url"]
 auth = ["__auth", "reqwest?/rustls-tls"]
 auth-tls-no-provider = ["auth", "reqwest?/rustls-tls-no-provider"]
+schemars = ["dep:schemars"]
 
 [dev-dependencies]
 tokio = { version = "1", features = ["full"] }
@@ -131,3 +132,8 @@ name = "test_message_protocol"
 required-features = ["client"]
 path = "tests/test_message_protocol.rs"
 
+[[test]]
+name = "test_message_schema"
+required-features = ["server", "client", "schemars"]
+path = "tests/test_message_schema.rs"
+