feat(llm): add built-in Google Gemini provider preset

[chethanuk]

User description

Description

Adds a built-in provider preset for Google Gemini via its OpenAI-compatible endpoint (https://generativelanguage.googleapis.com/v1beta/openai), so OpenCodeReview can run reviews against Gemini models with no new client, SDK, or protocol — it routes through the existing OpenAIClient path, mirroring the other 14 non-Anthropic presets.

Includes one piece of real logic: a new Provider.LegacyMaxTokens flag. Gemini's OpenAI-compat endpoint expects the legacy max_tokens field rather than max_completion_tokens (the SDK's default). The flag is threaded Provider → ResolvedEndpoint → ClientConfig → buildOpenAIParams so a provider: gemini user always sends max_tokens; every other preset is unchanged (defaults false).

Model list: ships the two GA models that are verified working end-to-end on OCR's tool-calling review path — gemini-2.5-flash (default) and gemini-2.5-pro.

Known limitation (deliberately out of scope): Gemini 3.x models are not shipped. Live testing showed every 3.x preview (gemini-3.1-pro-preview, gemini-3-flash-preview, gemini-3.1-flash-lite) returns HTTP 400 on OCR's multi-turn reviews: Gemini 3.x tool-call responses carry a google.thought_signature in extra_content that must be echoed back on the next turn, which the OpenAI-compat client does not yet do. They pass a plain llm test but break real reviews, so shipping them would be a footgun. Re-enabling 3.x would require thought_signature handling in the OpenAI-compat client — a separate change.

Note: a user who hand-configures Gemini via OCR_LLM_URL / the legacy llm block (instead of the gemini preset) still sends max_completion_tokens; the preset is the supported path. Gemini currently tolerates max_completion_tokens (no error), but the preset emits the documented max_tokens so the token budget is reliably honored.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally (make check: go mod tidy + go fmt + go vet + build + full test suite → check passed; 188 tests in internal/llm).
• Manual testing (described below)

Manual / live smoke testing against the real Gemini API:

1. Preset path — ocr config set provider gemini (+ GEMINI_API_KEY env-var fallback) → ocr llm test connects and ocr review --commit <hash> completes a real review. A pass-through logging proxy confirmed the request body carries max_tokens (not max_completion_tokens) on the wire, including on the tools=true review request.
2. Raw/direct path — OCR_LLM_URL=… OCR_USE_ANTHROPIC=false ocr llm test (bypasses the preset) sends max_completion_tokens, confirming the flag is what flips the field.
3. Model verification — all 5 candidate models were exercised on the tool-calling review path; only the two GA 2.5 models completed reviews cleanly, which is why the 3.x previews are excluded (see Known limitation above).

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (en/zh/ja provider tables)
• I have signed the CLA

Related Issues

Closes alibaba#49

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CodeAnt-AI Description

Add Gemini as a built-in provider and make its reviews work with the right token field

What Changed

• Added a built-in gemini provider that uses Google’s OpenAI-compatible endpoint and GEMINI_API_KEY
• Gemini now sends max_tokens instead of max_completion_tokens, which prevents review failures on that provider
• Limited the Gemini preset to the verified working models: gemini-2.5-flash and gemini-2.5-pro
• Documented the new provider in the configuration guides

Impact

✅ Gemini reviews run without token-field errors
✅ Fewer failed tool-calling reviews on Gemini
✅ Easier Gemini setup with a built-in preset

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9ed16cfb-3b02-442e-b062-5ed743e2b4b3

📥 Commits

Reviewing files that changed from the base of the PR and between e2d75b7 and 1bfbd83.

📒 Files selected for processing (9)

• internal/llm/client.go
• internal/llm/client_test.go
• internal/llm/providers.go
• internal/llm/providers_test.go
• internal/llm/resolver.go
• internal/llm/resolver_test.go
• pages/src/content/docs/en/configuration.md
• pages/src/content/docs/ja/configuration.md
• pages/src/content/docs/zh/configuration.md

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• Create PR with unit tests
• Commit unit tests in branch fix/issue-49-gemini-provider

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[gemini-code-assist]

Code Review

This pull request adds support for the Google Gemini API as an OpenAI-compatible provider. It introduces a LegacyMaxTokens configuration flag to allow endpoints like Gemini to use the legacy max_tokens parameter instead of max_completion_tokens. This flag is propagated from the provider presets to the OpenAI client, with corresponding unit tests and documentation updates. The feedback suggests extending this LegacyMaxTokens option to custom providers in the future, as they may also connect to endpoints requiring the legacy parameter.

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[gemini-code-assist]

Currently, LegacyMaxTokens is only set when resolving a preset provider (where isPreset is true). However, custom providers configured via custom_providers or the legacy llm block might also connect to OpenAI-compatible endpoints that require the legacy max_tokens parameter instead of max_completion_tokens. Consider adding a LegacyMaxTokens field (perhaps as a *bool to distinguish unset from false) to providerEntryConfig and llmFileConfig in a future update so that custom providers can also opt into using legacy max tokens if needed.

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