feat(streamable-http): add json_response option for stateless server mode (#683)

* feat(streamable-http): add json_response option for stateless server mode

Adds `json_response: bool` field to `StreamableHttpServerConfig`.
When true and `stateful_mode` is false, the server returns
`Content-Type: application/json` directly instead of `text/event-stream`,
eliminating SSE framing overhead for simple request-response patterns.

This completes server-side JSON response support (client-side was added
in #540) and contributes to the stateless server goals of SEP-1442 (#526).

Backwards-compatible: `json_response: false` (default) preserves all
existing SSE behaviour unchanged, and `stateful_mode: true` is unaffected.

Benchmark evidence (50 VUs, 5min, 2 CPUs):
- RPS: 770 → 1139 (+48%)
- get_user_cart latency: 41ms → 0.76ms (-98%)
- checkout latency: 41ms → 0.55ms (-99%)
- Zero regressions, zero errors

* fix(tower): add cancellation awareness and logging to JSON response path

* fix(test): add missing Default to StreamableHttpServerConfig in concurrent streams test

Made-with: Cursor

crates/rmcp/tests/test_sse_concurrent_streams.rs

@@ -84,6 +84,7 @@ async fn start_test_server(ct: CancellationToken, trigger: Arc<Notify>) -> Strin
             sse_keep_alive: Some(Duration::from_secs(15)),
             sse_retry: Some(Duration::from_secs(3)),
             cancellation_token: ct.child_token(),
+            ..Default::default()
         },
     );