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/model/serde_impl.rs

@@ -3,8 +3,8 @@ use std::borrow::Cow;
 use serde::{Deserialize, Serialize};
 
 use super::{
-    CustomNotification, Extensions, Meta, Notification, NotificationNoParam, Request,
-    RequestNoParam, RequestOptionalParam,
+    CustomNotification, CustomRequest, Extensions, Meta, Notification, NotificationNoParam,
+    Request, RequestNoParam, RequestOptionalParam,
 };
 #[derive(Serialize, Deserialize)]
 struct WithMeta<'a, P> {
@@ -249,6 +249,59 @@ where
     }
 }
 
+impl Serialize for CustomRequest {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        let extensions = &self.extensions;
+        let _meta = extensions.get::<Meta>().map(Cow::Borrowed);
+        let params = self.params.as_ref();
+
+        let params = if _meta.is_some() || params.is_some() {
+            Some(WithMeta {
+                _meta,
+                _rest: &self.params,
+            })
+        } else {
+            None
+        };
+
+        ProxyOptionalParam::serialize(
+            &ProxyOptionalParam {
+                method: &self.method,
+                params,
+            },
+            serializer,
+        )
+    }
+}
+
+impl<'de> Deserialize<'de> for CustomRequest {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let body =
+            ProxyOptionalParam::<'_, _, Option<serde_json::Value>>::deserialize(deserializer)?;
+        let mut params = None;
+        let mut _meta = None;
+        if let Some(body_params) = body.params {
+            params = body_params._rest;
+            _meta = body_params._meta.map(|m| m.into_owned());
+        }
+        let mut extensions = Extensions::new();
+        if let Some(meta) = _meta {
+            extensions.insert(meta);
+        }
+        Ok(CustomRequest {
+            extensions,
+            method: body.method,
+            params,
+        })
+    }
+}
+
 impl Serialize for CustomNotification {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where