fix(exec-server): suppress Windows taskkill output

[fcoury-oai]

Summary

This is the exec-server follow-up to #21759.

#21759 fixed the Windows taskkill output leak for the rmcp-client MCP teardown path, but #22050 showed that exec-server still had a parallel taskkill /T /F cleanup path in exec-server/src/connection.rs. Because that command inherited the parent stdio handles, Windows could still print SUCCESS: lines into the user's terminal during stdio child cleanup.

This change silences that remaining exec-server callsite by redirecting taskkill stdin, stdout, and stderr to Stdio::null().

What Changed

• add a Windows-only Stdio import in exec-server/src/connection.rs
• redirect the taskkill command in kill_windows_process_tree to Stdio::null() for stdin, stdout, and stderr
• keep the existing kill semantics unchanged by still checking .status() and preserving the existing fallback/logging behavior

How to Test

Manual validation is Windows-only, so I did not run the UI repro path locally here.

1. On Windows, use a Codex build from this branch.
2. Exercise an exec-server stdio flow that spawns a child process tree and then triggers transport cleanup.
3. Confirm the child process tree is still torn down.
4. Confirm the terminal no longer shows SUCCESS: The process with PID ... has been terminated. lines during cleanup.

Targeted tests:

• cargo test -p codex-exec-server client::tests::dropping_stdio_client_terminates_spawned_process -- --exact
• cargo test -p codex-exec-server client::tests::malformed_stdio_message_terminates_spawned_process -- --exact

Notes:

• cargo test -p codex-exec-server still hits unrelated local macOS sandbox-exec: sandbox_apply: Operation not permitted failures in tests/file_system.rs.

References

• Fixes the remaining callsite discussed in Windows: taskkill output leaks into TUI from exec-server stdio cleanup (parallel path to #20845) #22050
• Related earlier fix: fix(tui): suppress taskkill output for MCP teardown on Windows #21759