fix(exec-server): suppress Windows taskkill output (#22058)

## 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 #22050
- Related earlier fix: #21759

codex-rs/exec-server/src/connection.rs

@@ -1,3 +1,5 @@
+#[cfg(windows)]
+use std::process::Stdio;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
@@ -190,6 +192,9 @@ fn kill_windows_process_tree(pid: u32) -> bool {
     let pid = pid.to_string();
     match std::process::Command::new("taskkill")
         .args(["/PID", pid.as_str(), "/T", "/F"])
+        .stdin(Stdio::null())
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
         .status()
     {
         Ok(status) => status.success(),