fix: also catch BrokenResourceError from respond() for SHTTP terminate()

streamable_http's terminate() closes _write_stream_reader (the receive
end) before _write_stream (the send end). A handler reaching respond()
between those two closes gets BrokenResourceError (peer end closed)
rather than ClosedResourceError (our end closed). The stdio path only
ever hits ClosedResourceError because _receive_loop's async-with closes
the send end.

Author: maxisbey

src/mcp/server/lowlevel/server.py

@@ -494,11 +494,13 @@ async def _handle_request(
 
         try:
             await message.respond(response)
-        except anyio.ClosedResourceError:
+        except (anyio.BrokenResourceError, anyio.ClosedResourceError):
             # Transport closed between handler unblocking and respond. Happens
             # when _receive_loop's finally wakes a handler blocked on
             # send_request: the handler runs to respond() before run()'s TG
-            # cancel fires, but after _receive_loop closed _write_stream.
+            # cancel fires, but after the write stream closed. Closed if our
+            # end closed (_receive_loop's async-with exit); Broken if the peer
+            # end closed first (streamable_http terminate()).
             logger.debug("Response for %s dropped - transport closed", message.request_id)
             return