test: add parseSSE coverage (src/client/stream.ts)

[hell0gunawan]

抱歉，这使用的是中文和英文混合版本。

简介 / Summary

本 PR 为 src/client/stream.ts 中的 parseSSE 函数添加了完整的测试覆盖。

测试用例 / Test Cases

用例	描述
单个事件解析	解析包含 data 字段的单个 SSE 事件
多事件解析	顺序解析多个 SSE 事件
事件类型字段	解析 event: 字段
ID 字段	解析 id: 字段
全字段事件	解析同时包含 id、event、data 的事件
注释行跳过	以 : 开头的注释行被正确跳过
多行数据连接	多行 data 字段使用 \n 正确连接
无冒号行处理	格式错误的行（无冒号）被正确跳过
空数据字段	data: （无值）被正确处理
空响应体	null body 响应（204）不抛出错误
Reader Lock 释放	迭代完成后正确释放 reader lock

覆盖率 / Coverage

指标	之前	之后
Line Coverage	0%	100%
Branch Coverage	0%	100%

说明 / Notes

• 仅添加测试文件，不修改任何生产代码
• 所有 126 个现有测试保持通过
• 测试采用行为驱动方式，验证代码现有行为
• 无破坏性变更

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Summary

Adds 11 tests for parseSSE in src/client/stream.ts, bringing it from 0% to 100% line and branch coverage.

Why This Matters

parseSSE is core infrastructure used by all streaming commands (text chat, speech synthesize, video generation, etc.). Despite being a critical path, it had zero test coverage.

Test Cases

Case	Description
Single event parsing	Parses a single SSE event with data field
Multiple sequential events	Parses sequential SSE events correctly
Event type field	Parses event: field
ID field	Parses id: field
All-fields event	Parses event with id + event + data
Comment line skipping	Lines starting with : are skipped
Multi-line data concatenation	Multi-line data fields joined with \n
Missing-colon line handling	Malformed lines (no colon) are skipped
Empty data field	data: with no value handled correctly
Null body response	204 with null body does not throw
Reader lock release	Reader lock released after iteration

Coverage

Metric	Before	After
Line	0%	100%
Branch	0%	100%

Notes

• Only adds a test file — zero production code changes
• All 126 existing tests continue to pass
• Behavioral testing approach — validates existing behavior, no logic changes
• No breaking changes

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Note: This PR uses a Chinese + English bilingual description. If this causes inconvenience, please let me know and I will adjust.