fix: handle unbound response_or_error in send_request timeout

Initialize response_or_error to None before the try block and add an
explicit None check after. This prevents an UnboundLocalError when
anyio's fail_after cancel scope suppresses the timeout exception at
the exact deadline boundary without raising, leaving the variable
unassigned (see agronholm/anyio#589).

Github-Issue: #1717
Reported-by: maxisbey

Author: Bortlesboat

src/mcp/shared/session.py

@@ -270,6 +270,7 @@ async def send_request(
             # request read timeout takes precedence over session read timeout
             timeout = request_read_timeout_seconds or self._session_read_timeout_seconds
 
+            response_or_error: JSONRPCResponse | JSONRPCError | None = None
             try:
                 with anyio.fail_after(timeout):
                     response_or_error = await response_stream_reader.receive()
@@ -278,6 +279,15 @@ async def send_request(
                 message = f"Timed out while waiting for response to {class_name}. Waited {timeout} seconds."
                 raise MCPError(code=REQUEST_TIMEOUT, message=message)
 
+            if response_or_error is None:
+                raise MCPError(
+                    code=REQUEST_TIMEOUT,
+                    message=(
+                        f"Response not received for {request.__class__.__name__}. "
+                        "This may indicate a race condition in the cancel scope."
+                    ),
+                )
+
             if isinstance(response_or_error, JSONRPCError):
                 raise MCPError.from_jsonrpc_error(response_or_error)
             else: