feat: add google gemini provider

[jbj338033]

Summary

Adds goat-provider-gemini, a new provider crate that lets goat connect to Google Gemini models via either a Gemini API key or Google OAuth (Code Assist, gemini-cli compatible).

The provider is split into four modules: lib (Provider trait impl), wire (Gemini request/response format, thinking config, tool schema conversion), oauth (Google PKCE loopback flow with token refresh), and codeassist (Code Assist envelope + loadCodeAssist/onboardUser project onboarding).

Notable behavior: the OAuth path routes to cloudcode-pa.googleapis.com/v1internal (Code Assist backend) and resolves the GCP project via onboarding on first use. The API key path routes to the standard generativelanguage.googleapis.com/v1beta endpoint. Thinking config is model-generation-aware: 2.5 series uses thinkingBudget (integer), 3.x series uses thinkingLevel (enum).

Verification

• cargo fmt --all --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test --workspace — passed (24 new unit tests in wire.rs and oauth.rs)