feat(provider): add gemini google-generative-ai compatibility

[fuhao009]

Summary

• add provider compatibility groundwork for API mode and custom headers
• add Gemini (google-generative-ai) request/response compatibility
• support Gemini generateContent endpoint and tool-call mapping

Validation

• go build -o ./build/picoclaw ./cmd/picoclaw
• debug run confirms request path: /v1beta/models/{model}:generateContent
• upstream service may return 503 when account pool unavailable

[Leeaandrob]

@Zepan Gemini compatibility provider — useful addition. Same note as other provider PRs: PR #213 (Provider Refactor — roadmap #283) will restructure the provider layer.

Recommendation: Wait for #213 to merge. Gemini could be registered as an OpenAI-compatible provider in the new architecture, making integration simpler.

[yinwm]

Thanks for this contribution! The Gemini native API support is valuable.

However, this PR needs significant rework due to recent architecture changes:

1. Provider Architecture Refactored (feat(config): refactor provider architecture to protocol-based model_list #492)

PR #492 (merged 2026-02-20) introduced a protocol-based model_list configuration. See the design doc:
provider-refactoring.md

2. Current State

The existing gemini protocol uses OpenAI-compatible /chat/completions endpoint:
case "gemini":
return NewHTTPProviderWithMaxTokensField(...) // OpenAI compat

3. Suggested Approach

To add native Gemini API support, please:

1. Create a new protocol prefix (e.g., gemini-native or google-generative-ai)
2. Create pkg/providers/gemini/provider.go following the pattern of anthropic/ or openai_compat/
3. Register the new protocol in pkg/providers/factory_provider.go

Example configuration:

  {
    "model_list": [
      {
        "model_name": "my-gemini",
        "model": "gemini-native/gemini-2.0-flash",
        "api_key": "xxx"
      }
    ]
  }

@fuhao009

[xiaket]

@fuhao009 gentle nudge :)

[CLAassistant]

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[sipeed-bot]

@fuhao009 Hi! This PR adding Gemini provider compatibility has had no activity for over 2 weeks now, so I'm going to close it for housekeeping. If you're still working on this or want to pick it back up later, feel free to reopen anytime and we'll continue from here.