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/Cargo.toml

@@ -210,6 +210,11 @@ name = "test_streamable_http_priming"
 required-features = ["server", "client", "transport-streamable-http-server", "reqwest"]
 path = "tests/test_streamable_http_priming.rs"
 
+[[test]]
+name = "test_streamable_http_json_response"
+required-features = ["server", "client", "transport-streamable-http-server", "reqwest"]
+path = "tests/test_streamable_http_json_response.rs"
+
 
 [[test]]
 name = "test_custom_request"