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.

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