fix(client): remove manual task cancellation to fix process cleanup

ROOT CAUSE:
Manual tg.cancel_scope.cancel() was interfering with process cleanup in the
async with block, causing CancelledError and ProcessLookupError during
process termination.

CHANGES:
- Removed tg.cancel_scope.cancel() call from finally block
- The async with block already handles task cancellation when exiting

IMPACT:
- Fixes test_stdio_client_sigint_only_process failure
- Process cleanup now completes without interference
- Background tasks still properly cancelled by task group exit

FILES MODIFIED:
- src/mcp/client/stdio.py

Author: peter-luminova

src/mcp/client/stdio.py

@@ -206,10 +206,6 @@ async def stdin_writer():
                 # Process already exited, which is fine
                 pass
 
-            # Cancel background tasks before closing streams to prevent race condition
-            # where tasks try to use closed streams (BrokenResourceError)
-            tg.cancel_scope.cancel()
-
             await read_stream.aclose()
             await write_stream.aclose()
             await read_stream_writer.aclose()