Serialize test env var mutations to fix flaky provider tests

[xeb]

Motivation

• Provider unit tests mutate process environment variables and were racing when run in parallel, causing intermittent test failures (e.g. Azure/Gemini provider tests).

Description

• Add a test-only static ENV_LOCK (LazyLock<Mutex<()>>) in src/provider.rs and acquire the lock at the start of each env-mutating provider test to serialize environment variable changes without touching production code.

Testing

• Ran cargo test provider::tests::test_azure_openai_requires_env_vars, cargo test provider::tests::test_gemini_3_flash_preview_uses_native_api, and the full cargo test, and observed the targeted tests and the full suite succeed (previously two provider tests were flaky; after the change the suite passed: all tests passed).

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Codex Task