feat: add support for custom requests (#590)

#580 and #556 introduced support for custom notifications,
so this PR takes the next logical step and adds support for custom requests:

- Introduces `CustomRequest` and `CustomResult` model types, wires them into the client/server
  request and result unions, and allows `ClientRequest::method()` to return the dynamic method
  name.
- Implements serde and meta handling for `CustomRequest` so `_meta` is carried through
  extensions; adds default `on_custom_request` handlers that return `METHOD_NOT_FOUND` unless
  overridden.
- Updates JSON schema fixtures to include the new request/result shapes and `EmptyObject`
  strictness.
- Adds tests for custom request roundtrips and end-to-end client↔server handling.
- Focused integration test in `crates/rmcp/tests/test_custom_request.rs`.

For additional testing, I used this locally to update Codex to use a custom
request instead of a custom notification so that it gets an "ack" from the MCP
server to ensure it has processed the update before sending more messages:
https://github.com/openai/codex/pull/8142.

crates/rmcp/src/handler/server.rs

@@ -69,6 +69,10 @@ impl<H: ServerHandler> Service<RoleServer> for H {
                 .list_tools(request.params, context)
                 .await
                 .map(ServerResult::ListToolsResult),
+            ClientRequest::CustomRequest(request) => self
+                .on_custom_request(request, context)
+                .await
+                .map(ServerResult::CustomResult),
         }
     }
 
@@ -200,6 +204,19 @@ pub trait ServerHandler: Sized + Send + Sync + 'static {
     ) -> impl Future<Output = Result<ListToolsResult, McpError>> + Send + '_ {
         std::future::ready(Ok(ListToolsResult::default()))
     }
+    fn on_custom_request(
+        &self,
+        request: CustomRequest,
+        context: RequestContext<RoleServer>,
+    ) -> impl Future<Output = Result<CustomResult, McpError>> + Send + '_ {
+        let CustomRequest { method, .. } = request;
+        let _ = context;
+        std::future::ready(Err(McpError::new(
+            ErrorCode::METHOD_NOT_FOUND,
+            method,
+            None,
+        )))
+    }
 
     fn on_cancelled(
         &self,