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.
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