`resolve_api_key_source_reports_env_when_set` and
`resolve_api_key_source_prefers_config_over_env` both mutate
DEEPSEEK_API_KEY in process-global env. With cargo test's default
parallelism they race — one test reads while the other's set is still
active — causing intermittent CI failures on Linux (passes locally).
Fix: module-level `static ENV_LOCK: Mutex<()>`, both tests acquire
before touching env. `unwrap_or_else(|p| p.into_inner())` recovers
from poisoning so a panic in one test doesn't cascade.
Closes the CI failure introduced in the v0.8.9 cut (4511ea76); does
not affect runtime behavior — `Config::default()` is still empty and
`resolve_api_key_source` semantics are unchanged.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
d06eaed008