Hardens PTY Python REPL test and make MCP test startup deterministic
**Summary**
- `utils/pty/src/tests.rs`
- Added a REPL readiness handshake (`wait_for_python_repl_ready`) that
repeatedly sends a marker and waits for it in PTY output before sending
test commands.
- Updated `pty_python_repl_emits_output_and_exits` to:
- wait for readiness first,
- preserve startup output,
- append output collected through process exit.
- Reduces Windows/ConPTY flakiness from early stdin writes racing REPL
startup.
- `mcp-server/tests/suite/codex_tool.rs`
- Avoid remote model refresh during MCP test startup, reducing
timeout-prone nondeterminism.
This PR changes stdio MCP child processes to run in their own process
group
* Add guarded teardown in codex-rmcp-client: send SIGTERM to the group
first, then SIGKILL after a short grace period.
* Add terminate_process_group helper in process_group.rs.
* Add Unix regression test in process_group_cleanup.rs to verify wrapper
+ grandchild are reaped on client drop.
Addresses reported MCP process/thread storm: #10581
Unified exec isn't working on Linux because we don't provide the correct
arg0.
The library we use for pty management doesn't allow setting arg0
separately from executable. Use the same aliasing strategy we use for
`apply_patch` for `codex-linux-sandbox`.
Use `#[ctor]` hack to dispatch codex-linux-sandbox calls.
Addresses https://github.com/openai/codex/issues/6450
