fix(relay): reject image models on chat completions

[sl999ops]

⚠️ 提交说明 / PR Notice

📝 变更描述 / Description

修复图片生成模型可通过 /v1/chat/completions 被当作文本补全转发和计费的问题。

• 在文本请求校验阶段拒绝图片生成模型访问 Chat Completions，返回明确的 400 错误并提示使用 /v1/images/generations。
• 将 gpt-image-* 识别改为前缀匹配，覆盖 gpt-image-2、模型快照和后续版本；同时覆盖 chatgpt-image-latest 与 DALL·E。
• 增加回归测试，验证图片模型被拒绝、普通文本模型仍可通过。

校验发生在 token 估算、预扣费和上游转发之前，避免错误端点请求造成额度消耗。

🚀 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

Related to #6308.

✅ 提交前检查项 / Checklist

• 人工确认: 已人工确认
• 非重复提交: 已检查关联 Issue 与现有实现。
• Bug fix 说明: 修复公共 Relay 请求中图片模型与 Chat Completions 端点不兼容的问题。
• 变更理解: 校验在计费和转发前执行，且仅影响已识别的图片生成模型。
• 范围聚焦: 仅包含模型分类、入口校验和回归测试。
• 本地验证: go test ./relay/helper -run 'TestChatCompletionsRejectsImageGenerationModels|TestChatCompletionsAcceptsTextModel' -count=1 通过；其余后端包测试也已通过。
• 安全合规: 未包含凭据或用户数据。

📸 运行证明 / Proof of Work

修复后，以下请求会在 Relay 入口返回 400，而不会选择渠道或扣费：

POST /v1/chat/completions
{ "model": "gpt-image-2", "stream": true, "messages": [{"role":"user","content":"Generate an image"}] }

错误信息：model "gpt-image-2" is not supported on /v1/chat/completions; use /v1/images/generations instead

📸 运行证明 / Proof of Work

使用 gpt-image-2 请求 /v1/chat/completions，现在在 Relay 入口返回 400，
未进入上游转发或计费流程。

本地回归测试：

ok github.com/QuantumNous/new-api/relay/helper

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

## Summary by CodeRabbit

- **Bug Fixes**
  - Improved recognition of “response-only” OpenAI image model identifiers (including a broader image prefix pattern and the latest image model name).
  - Requests to **POST /v1/chat/completions** using image-generation models are now rejected with guidance to use **POST /v1/images/generations** instead.
- **Tests**
  - Added coverage to confirm chat-completions rejects image-generation models and accepts text models as expected.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

[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: 49e4db53-c916-4f14-88e1-a8ebb22e8ec8

📥 Commits

Reviewing files that changed from the base of the PR and between 1186b91 and 4734f58.

📒 Files selected for processing (3)

• common/model.go
• relay/helper/text_endpoint_compatibility_test.go
• relay/helper/valid_request.go

🚧 Files skipped from review as they are similar to previous changes (3)

• relay/helper/valid_request.go
• common/model.go
• relay/helper/text_endpoint_compatibility_test.go

---

Walkthrough

Image-generation model patterns now include current image model variants. Chat-completions validation rejects image-generation models with guidance to use the images endpoint, while text-model requests continue to succeed.

Changes

Image model endpoint validation

Layer / File(s)	Summary
Image model classification common/model.go	Updates response-only model patterns to match gpt-image-* names and chatgpt-image-latest.
Chat-completions validation and tests relay/helper/valid_request.go, relay/helper/text_endpoint_compatibility_test.go	Rejects image-generation models on chat completions, directs callers to the images endpoint, and verifies text-model acceptance.

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

Possibly related PRs

• QuantumNous/new-api#6423: Updates image-model detection and endpoint mapping related to chat-completions validation.

Poem

A rabbit checked the model line,
Image chats now hop to a proper sign.
Text models pass without delay,
While image requests find their way.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: rejecting image models on chat completions.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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