fix: add warning log for unknown/expired session ID in _handle_stateful_request

When a request arrives with an unknown or expired session ID, the
server now logs a warning before returning 404. This helps operators
diagnose stale session issues in production.

Closes #2204

Author: Alberto Farah

src/mcp/server/streamable_http_manager.py

@@ -272,6 +272,10 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
             # Unknown or expired session ID - return 404 per MCP spec
             # TODO: Align error code once spec clarifies
             # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
+            logger.warning(
+                "Rejected request with unknown or expired session ID: %s",
+                session_id,
+            )
             error_response = JSONRPCError(
                 jsonrpc="2.0",
                 id=None,