Fix FallbackProcess cleanup to not terminate process

- Remove process termination from FallbackProcess.__aexit__
- Only handle stream cleanup in __aexit__, not process termination
- Process termination is now handled by stdio_client's cleanup sequence

This separation of concerns ensures proper MCP spec-compliant shutdown
flow where the stdio_client controls the termination sequence, not the
process wrapper's context manager.

Reported-by: Felix Weinberger

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: felixweinberger

src/mcp/client/stdio/win32.py

@@ -73,10 +73,11 @@ async def __aexit__(
         exc_val: BaseException | None,
         exc_tb: object | None,
     ) -> None:
-        """Terminate and wait on process exit inside a thread."""
-        self.popen.terminate()
-        await to_thread.run_sync(self.popen.wait)
+        """Clean up streams without terminating the process.
 
+        The process termination is handled by stdio_client's cleanup sequence
+        which implements the MCP spec-compliant shutdown flow.
+        """
         # Close the file handles to prevent ResourceWarning
         if self.stdin:
             await self.stdin.aclose()