合入两处 Jules 改进，并补齐评审发现的缺口
User description
合入 #2275 与 #2276 两处 Jules 改进，并补齐评审发现的缺口。
一是 YAML 行扫描：加冒号前置检查、行内集合名改借用切片；评审一致指出的二次方开销问题已修：跟踪集合改回哈希表（借用切片保留），笔记里不实的“16 个上限”“零分配”说法也已纠正。
二是订阅 User-Agent 校验：入口与 HTTP 客户端两处都拦长度与控制字符；评审指出的两处真问题已修：被后端拒绝的值不再静默留在本地冒充生效（恢复后端现值并提示），长度报错里的“512 字符”改成“512 字节”以符实际按字节计的口径。
另：#2275 原 PR 上 Sonar 质量门报新代码可靠性 C 级，具体是哪条规则未从现有信息里确认，本 PR 的 CI 会重跑 Sonar，届时再看。
CodeAnt-AI Description
Validate subscription User-Agents and speed up YAML scanning
What Changed
- Custom subscription User-Agents are trimmed and rejected if they exceed 512 bytes or contain control characters.
- When a value is rejected, settings revert to the saved User-Agent and show an error; subscription requests use the default User-Agent if a saved value is invalid.
- YAML scanning skips unnecessary checks on lines without mapping separators while keeping large anchor lookups efficient.
Impact
✅ Fewer failed subscription downloads from invalid User-Agents
✅ Clearer custom User-Agent rejection
✅ Faster scans of YAML lines without mapping separators
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