- relay: SSE stream error exposure improvements (Claude/Gemini/OpenAI)
- test: add mock capture server for E2E relay capture testing

Co-Authored-By: Claude <noreply@anthropic.com>

This reverts commit ff14caed7c.

- ClaudeRequest 新增 CacheControl 字段
- RequestOpenAI2ClaudeMessage 自动注入顶层 cache_control: {"type":"ephemeral"}
- System 复合内容和普通消息 content block 的 CacheControl 透传
- ParseContent 新增 []MediaContent 类型断言，修复透传路径
- 新增 8 个单元测试覆盖自动注入、透传、序列化等场景

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>