Add a process-scoped Mutex that all env-var-mutating tests in
openai_codex::tests must hold. This prevents std::env::set_var /
remove_var calls from racing when Rust's test harness runs them on
parallel threads.
Affected tests:
- resolve_responses_url_prefers_explicit_endpoint_env
- resolve_responses_url_uses_provider_api_url_override
- resolve_reasoning_effort_prefers_configured_override
- resolve_reasoning_effort_uses_legacy_env_when_unconfigured
argenis de la rosa
2479b17523