fix(config): restore huggingface env precedence

[gaord]

This restores the Hugging Face API key env precedence on the TUI config surface so scripts/check-provider-registry.py (the CI/Lint gate) passes again on main.

Background

• crates/tui/src/config.rs previously expressed the precedence through a helper (provider_env_api_key), but the registry lint script checks for the literal std::env::var("HUGGINGFACE_API_KEY") / std::env::var("HF_TOKEN") strings in this file and fails when it cannot find them in the expected order.
• That drift is unrelated to the work in feat(tui): preserve thinking/tool blocks when seeding thread from session #3300 (session/thread process preservation), so it is being fixed here on its own branch off the latest main.

What changed

• provider_env_api_key() now spells out the Hugging Face env order explicitly: HUGGINGFACE_API_KEY first, falling back to HF_TOKEN.
• Behavior on the runtime side is unchanged: HF_TOKEN alone still works, HUGGINGFACE_API_KEY still wins when both are set, empty values are ignored, and the rest of the providers continue to flow through the existing helper path.
• The Hugging Face config tests (huggingface_hf_token_env_api_key_resolves, huggingface_short_env_fallbacks_configure_route, huggingface_env_overrides_key_base_url_and_model, huggingface_missing_key_error_mentions_env_fallbacks, etc.) still pass.

Validation

• python3 scripts/check-provider-registry.py now reports Provider registry drift check passed.
• cargo test -p codewhale-tui huggingface_ is green (6 passed).

Out of scope

• The failing tools::shell tests on windows-latest for feat(tui): preserve thinking/tool blocks when seeding thread from session #3300 are unrelated to this file and are tracked separately.

[greptile-apps]

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[gemini-code-assist]

Code Review

This pull request refactors the Hugging Face API key environment variable resolution in crates/tui/src/config.rs. It replaces direct checks of HF_TOKEN with a call to provider_env_api_key, which has been updated to explicitly handle the precedence of HUGGINGFACE_API_KEY over HF_TOKEN while ensuring the values are non-empty. Additionally, some import statements have been sorted alphabetically. There are no review comments to address, and I have no further feedback to provide.

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