Fix Windows ResourceWarning by properly closing process streams

The new stdio cleanup sequence was causing ResourceWarning on Windows CI
about unclosed _ProactorReadPipeTransport objects. This was because we
were terminating processes while their stdout streams were still open.

Changes:
- Close read_stream_writer and write_stream_reader first to signal tasks
- Explicitly close process.stdout before terminating the process
- This ensures Windows ProactorEventLoop properly cleans up pipe transports

The warnings were surfacing in unrelated tests due to garbage collection
timing, but the root cause was incomplete cleanup in stdio_client.

Author: felixweinberger

src/mcp/client/stdio/__init__.py

@@ -184,13 +184,26 @@ async def stdin_writer():
             # 1. Close input stream to server
             # 2. Wait for server to exit, or send SIGTERM if it doesn't exit in time
             # 3. Send SIGKILL if still not exited
+
+            # First, close our streams to signal the reader/writer tasks to stop
+            await read_stream_writer.aclose()
+            await write_stream_reader.aclose()
+
             if process.stdin:
                 try:
                     await process.stdin.aclose()
                 except Exception:
                     # stdin might already be closed, which is fine
                     pass
 
+            # Close stdout to ensure the ProactorReadPipeTransport is properly closed
+            if process.stdout:
+                try:
+                    await process.stdout.aclose()
+                except Exception:
+                    # stdout might already be closed, which is fine
+                    pass
+
             try:
                 # Give the process time to exit gracefully after stdin closes
                 with anyio.fail_after(2.0):
@@ -211,8 +224,6 @@ async def stdin_writer():
 
             await read_stream.aclose()
             await write_stream.aclose()
-            await read_stream_writer.aclose()
-            await write_stream_reader.aclose()
 
 
 def _get_executable_command(command: str) -> str:

src/mcp/client/stdio/win32.py

@@ -158,8 +158,7 @@ async def create_windows_process(
     except Exception as e:
         # If anyio failed for other reasons, try without creation flags
         logger.info(
-            f"anyio.open_process failed with creation flags, "
-            f"retrying without flags for command: {command}. Error: {e}"
+            f"anyio.open_process failed with creation flags, retrying without flags for command: {command}. Error: {e}"
         )
         process = await anyio.open_process(
             [command, *args],
@@ -210,7 +209,7 @@ def _create_windows_fallback_process(
     except Exception as e:
         # If creationflags failed, try without them
         logger.info(
-            f"FallbackProcess with creation flags failed, " f"retrying without flags for command: {command}. Error: {e}"
+            f"FallbackProcess with creation flags failed, retrying without flags for command: {command}. Error: {e}"
         )
         popen_obj = subprocess.Popen(
             [command, *args],