feat: add gemini-3-pro-image and gemini-3.1-flash-image GA models

[feitianbubu]

⚠️ 提交说明 / PR Notice

Google 已于 2026-07-17 停用 gemini-3-pro-image-preview 与 gemini-3.1-flash-image-preview 端点，GA 替代名为 gemini-3-pro-image 与 gemini-3.1-flash-image（参见 官方迁移说明）。

本 PR 注册这两个 GA 模型名：

• relay/channel/gemini/constant.go：加入渠道默认 ModelList（Vertex 渠道复用该列表，一并生效）。
• setting/model_setting/gemini.go：加入 SupportedImagineModels 默认值，使 GA 名走图像生成分支（responseModalities: ["TEXT","IMAGE"]）。

无需其他改动：补全倍率的 gemini-3-pro-image 前缀规则天然覆盖 GA 名；preview 名暂保留以兼容存量渠道配置。

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix)
• ✨ 新功能 (New feature)
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• 无

✅ 提交前检查项 / Checklist

• 人工确认: 我已亲自整理并撰写此描述，没有直接粘贴未经处理的 AI 输出。
• 非重复提交: 我已搜索现有的 Issues 与 PRs，确认不是重复提交。
• Bug fix 说明: 若此 PR 标记为 Bug fix，我已提交或关联对应 Issue，且不会将设计取舍、预期不一致或理解偏差直接归类为 bug。
• 变更理解: 我已理解这些更改的工作原理及可能影响。
• 范围聚焦: 本 PR 未包含任何与当前任务无关的代码改动。
• 本地验证: 已在本地运行并通过测试或手动验证，维护者可以据此复核结果。
• 安全合规: 代码中无敏感凭据，且符合项目代码规范。

Summary by CodeRabbit

• New Features
  • Added support for the Gemini 3 Pro Image model.
  • Updated the available Gemini image model options to include the new model.

[coderabbitai]

Walkthrough

The Gemini model registry and default imagine-model settings now include gemini-3-pro-image, alongside the existing Gemini image models.

Changes

Gemini image model availability

Layer / File(s)	Summary
Register and enable Gemini image model relay/channel/gemini/constant.go, setting/model_setting/gemini.go	The stable Gemini model list and default supported imagine-model configuration now include gemini-3-pro-image.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Poem

A rabbit saw a model bloom,
An image spark lit up the room.
The Gemini list grew bright,
New pictures hopped in sight.
“What a tidy update!” it cheered.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding GA Gemini image models.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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