fix(server): return 405 on GET/DELETE in stateless HTTP mode

[faridun-ag2]

Summary

In stateless HTTP mode (stateless_http=True), GET requests to the MCP endpoint were creating a transport and opening an SSE stream that could never receive server-initiated messages, idling until timeout. DELETE had the same shape (no session to terminate). This wastes connections, especially on serverless platforms (Cloud Run, Lambda).

This PR makes StreamableHTTPSessionManager._handle_stateless_request short-circuit GET and DELETE with HTTP 405 before any transport is spawned. POST is unchanged. Stateful mode is unchanged.

The MCP spec permits this: "Servers MAY return HTTP 405 Method Not Allowed if an SSE stream is not offered at the endpoint." The TypeScript SDK already implements this behavior.

Closes #2474.

What changed

• src/mcp/server/streamable_http_manager.py — early-return guard at the top of _handle_stateless_request. If request.method is GET or DELETE, return a JSON-RPC formatted 405 with Allow: POST and bail out before transport creation.
• tests/server/test_streamable_http_manager.py — three new tests:
  • test_stateless_get_returns_405 — status, Allow header, JSON-RPC error body
  • test_stateless_delete_returns_405 — same checks for DELETE
  • test_stateless_get_does_not_create_transport — asserts no StreamableHTTPServerTransport is instantiated for a stateless GET

Design notes

• Layer placement. The guard lives in the manager, not the transport. The manager already owns the stateless/stateful routing decision; StreamableHTTPServerTransport stays mode-agnostic.
• Response format. Mirrors the existing 404 "Session not found" handler (JSONRPCError(jsonrpc="2.0", id=None, error=ErrorData(code=INVALID_REQUEST, ...)) serialized via model_dump_json(by_alias=True, exclude_unset=True)), with an added Allow: POST header.
• Allow header. Returns Allow: POST (not GET, POST, DELETE like the stateful transport's _handle_unsupported_request). The difference is intentional — stateless mode genuinely only supports POST.
• HEAD/OPTIONS in stateless. Not caught by this guard; they fall through to the transport's _handle_unsupported_request and still return 405. The Allow header from that path advertises GET/DELETE that stateless doesn't actually support — minor inconsistency, happy to extend the guard if reviewers prefer.
• No new imports. Request, Response, HTTPStatus, INVALID_REQUEST, ErrorData, JSONRPCError were all already imported in this module.

Test plan

• uv run --frozen pytest tests/server/test_streamable_http_manager.py — 14/14 pass
• uv run --frozen pytest tests/ -k stateless — 13/13 pass
• uv run --frozen pytest tests/server/ — 477/477 pass (no regressions)
• uv run --frozen ruff format + ruff check — clean
• uv run --frozen pyright — 0 errors
• Targeted coverage on streamable_http_manager.py — 100% (branch-covered)
• strict-no-cover — clean
• pre-commit run on changed files — all applicable hooks pass