test: Add lock to fix env conflict in test

[hongqitai]

Summary

The test direct_provider_ignores_foreign_deepseek_root_default_model will read env DEEPSEEK_BASE_URL.
But deepseek_base_url_env_scopes_to_self_hosted_providers will write it and cause test failed.
Use lock_test_env get lock before test in direct_provider_ignores_foreign_deepseek_root_default_model.

Testing

• [ x ] cargo fmt --all -- --check
• [ x ] cargo clippy --workspace --all-targets --all-features
• [ x ] cargo test --workspace --all-features

Checklist

• [ x ] Updated docs or comments as needed
• [ x ] Added or updated tests where relevant
• [ x ] Verified TUI behavior manually if UI changes
• [ x ] Harvested/co-authored credit uses a GitHub numeric noreply address

[Hmbown]

Correct fix: direct_provider_ignores_foreign_deepseek_root_default_model reads DEEPSEEK_BASE_URL while a sibling test writes it; taking lock_test_env() serializes them. Matches the established pattern in this file. CI green across all three OSes. Thanks @hongqitai.

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