fix(relay): remove extra blank line from Claude SSE stream

[zhangzhichaolove]

⚠️ 提交说明 / PR Notice

Important

• 请提供人工撰写的简洁摘要，避免直接粘贴未经整理的 AI 输出。

📝 变更描述 / Description

修复 Claude 原生 /v1/messages 流式转发时，SSE event 之间多输出一个空行的问题。

问题原因是 ClaudeChunkData 在 data: 行尾主动追加了 \n，而 common.CustomEvent 在写出 data: 字段时还会自动补上标准 SSE event 终止符 \n\n，最终导致输出变成 data: ...\n\n\n。本次改动移除了 ClaudeChunkData 中的多余换行，让 Claude stream event 恢复为标准的单个空行分隔。

同时新增回归测试，连续写出两个 Claude SSE chunk，断言输出不包含三连换行，避免后续再次引入同类格式回归。

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix) - 请关联对应 Issue，避免将设计取舍、理解偏差或预期不一致直接归类为 bug
• ✨ 新功能 (New feature) - 重大特性建议先通过 Issue 沟通
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• Closes Bug: Claude /v1/messages stream relay emits an extra blank line between SSE events #5402

✅ 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。

📸 运行证明 / Proof of Work

go test ./relay/helper
go test ./relay/channel/claude
go test ./relay/...

Summary by CodeRabbit

• Bug Fixes
  • Fixed streaming event formatting so SSE payloads no longer include an extra blank line.
  • Improved response formatting consistency for message streaming, preventing malformed line breaks in event output.
• Tests
  • Added coverage to verify streamed events are formatted correctly and do not contain unintended extra newlines.

[coderabbitai]

Walkthrough

Claude SSE chunk rendering now omits the trailing newline on the data: line, and a new unit test checks that two Claude events produce a single blank line between frames without triple-newline sequences.

Changes

Claude SSE blank-line fix

Layer / File(s)	Summary
SSE chunk formatting relay/helper/common.go	ClaudeChunkData formats the data: line without a trailing newline.
Output regression test relay/helper/common_test.go	A new test renders two Claude events and asserts the SSE body matches the expected framing without \n\n\n.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I hopped through streams with careful feet, 🐇
Fixed one blank line so things look neat.
No triple-newline puddles remain,
Just tidy chunks in SSE rain.
Hop hop — the relay sings again!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: removing an extra blank line from the Claude SSE stream.
Linked Issues check	✅ Passed	The code removes the trailing newline in ClaudeChunkData and adds regression coverage, matching issue #5402's requirements.
Out of Scope Changes check	✅ Passed	The PR stays focused on the SSE formatting fix and a targeted regression test with no unrelated changes.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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