fix(vertex): convert Anthropic Messages to Gemini for Gemini upstreams

[zkasuran]

📝 变更描述 / Description

Vertex 渠道当上游模型为 Gemini 时，调用 Anthropic Messages 接口 POST /v1/messages
会返回 HTTP 400（Vertex 报 Unknown name anthropic_version / messages / max_tokens）。

原因在 relay/channel/vertex/adaptor.go 的 ConvertClaudeRequest：它无论上游是
Claude 还是 Gemini，都把请求包装成 VertexAIClaudeRequest（anthropic_version +
messages + max_tokens）。但 Gemini 上游走的是 generateContent URL，需要的是
contents + generationConfig，所以被 Vertex 拒绝。/v1/chat/completions 路径没有这个
问题，因为 ConvertOpenAIRequest 已经按 RequestMode 分支，对 Gemini 调用
service.ConvertRequest(..., RelayFormatGemini, ...)。只有 /v1/messages 入口缺了这个分支。

本 PR 在 ConvertClaudeRequest 补上同样的分支：当 RequestMode == RequestModeGemini
时，用已注册的 claude_messages_to_gemini_generate_content 转换器（经
service.ConvertRequest）把 Anthropic Messages 请求转成 Gemini generateContent，返回
*dto.GeminiChatRequest；Claude 上游保持原有 Anthropic 包装不变。响应侧无需改动：
RelayFormatClaude 下共享的 gemini.GeminiChatHandler 已经把 Gemini 响应转回 Claude
Messages 格式。

关于渠道测试误报成功：渠道测试对 Anthropic 端点走的是同一个 adaptor.ConvertClaudeRequest，
并且会检查上游状态码与错误体。它之所以“成功”，是因为默认渠道测试用的是 OpenAI 端点
（/v1/chat/completions），那条路径转换本就正确，从未触发 /v1/messages 的缺陷。修复后，
对 Anthropic 端点做渠道测试会经过同一段修正后的转换逻辑，产生真正的 Gemini 请求体，不再掩盖问题。
因此测试代码本身无需改动。

🚀 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

• Closes [Bug] Vertex AI Gemini 的 /v1/messages 请求未转换，渠道测试却误报转换成功 #6715

✅ 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。

AI disclosure

This change was written with AI assistance (Claude). I reviewed the code and the
Vertex/Anthropic request shapes and verified it locally. Verified before
submitting: go build ./... (exit 0), go vet ./relay/channel/vertex/...
(clean), go test ./relay/channel/vertex/... (pass, including the new
regression test). The author is not a repo core developer, so this disclosure is
included per contribution policy.

📸 运行证明 / Proof of Work

New regression test TestConvertClaudeRequestByRequestMode fails before the fix
and passes after.

Before the fix (fix reverted, test kept):

--- FAIL: TestConvertClaudeRequestByRequestMode/gemini_upstream_converts_to_generateContent
    map[anthropic_version:vertex-2023-10-16 max_tokens:16 messages:[...]] should not contain "max_tokens"
    expected *dto.GeminiChatRequest, got *vertex.VertexAIClaudeRequest
FAIL

After the fix:

=== RUN   TestConvertClaudeRequestByRequestMode
--- PASS: TestConvertClaudeRequestByRequestMode (0.00s)
    --- PASS: TestConvertClaudeRequestByRequestMode/gemini_upstream_converts_to_generateContent (0.00s)
    --- PASS: TestConvertClaudeRequestByRequestMode/claude_upstream_keeps_anthropic_messages_body (0.00s)
PASS
ok  	github.com/QuantumNous/new-api/relay/channel/vertex	0.012s

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 579929c6-8ea2-4b4b-a551-68fa2cae266c

📥 Commits

Reviewing files that changed from the base of the PR and between ccd535e and aa46c8e.

📒 Files selected for processing (2)

• relay/channel/vertex/adaptor.go
• relay/channel/vertex/adaptor_test.go

---

Walkthrough

The Vertex adaptor now converts Claude requests to Gemini requests when Gemini mode is active. Tests verify Gemini conversion and preserve Claude request behavior.

Changes

Vertex request conversion

Layer / File(s)	Summary
Request conversion and regression coverage relay/channel/vertex/adaptor.go, relay/channel/vertex/adaptor_test.go	Gemini-mode Claude requests now use service.ConvertRequest, validate the converted DTO type, set request_model, and return conversion errors. Tests verify Gemini fields and Claude metadata separately.

Estimated code review effort: 2 (Simple) | ~10 minutes

Mergeability Score: ⚪ Minimal · up to aa46c

This localized request-conversion change is merge-ready after normal checks and review; no actionable merge-blocking risk remains.

Possibly related PRs

• QuantumNous/new-api#5825: Updates Vertex Claude request conversion with service.ConvertRequest and result type validation.
• QuantumNous/new-api#6698: Modifies Claude/Gemini request conversion and related tests.

Suggested reviewers: calcium-ion

Poem

A rabbit saw Gemini hop,
While Claude requests changed their stop.
Contents bloom, tokens align,
Anthropic fields stay in line.
Tests guard each careful way,
Conversion works today.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR fixes Claude-to-Gemini request conversion and adds regression tests, but it does not update the Anthropic channel test to use dto.ClaudeRequest and the production path [#6715].	Update the Anthropic channel test to construct dto.ClaudeRequest and execute ConvertClaudeRequest through the same production conversion path.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The changes are limited to Vertex request conversion and related regression tests, which match the linked issue objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes converting Anthropic Messages requests to Gemini for Vertex Gemini upstreams.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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