optimize(rpctimeout): defense user provided non-compliant ctx with ctx.Done() triggered but ctx.Err() returning nil

[DMwangnima]

What type of PR is this?

optimize

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• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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• 背景 / 问题
  当用户传入的 ctx 违反 context.Context 约定——Done() 已触发但 Err() 仍返回 nil（常见于自定义 "WithoutCancel" wrapper：embed 了父 context 导致继承其 Done()，却重写 Err() 为 nil）。会在 RPC 超时路径上引发 nil pointer panic：

1. timeoutTask.Wait() 中 Cancel(parentErr=nil) 成为空操作，timeoutContext.ch 未关闭、Err() 仍为 nil
2. Wait() 返回 (ctx, nil) → Call() 视为成功 → 设置 recycleRI=true → PutRPCInfo 回收 RPCInfo（ri.to 被清零）
3. worker goroutine 仍在运行 → 访问 ri.To().ServiceName() → nil panic
4. Run() 的 recover 中 ClientPanicToErr 再次调用 ri.To() → double panic

• 方案

1. handleParentDone 在 Cancel 前检查 parentErr，为 nil 时用哨兵错误 errUserProvidedContextNonCompliant 替代，保证 Cancel 始终收到非 nil 错误、ch 必然关闭
2. makeTimeoutErr 识别该哨兵错误，直接返回 ErrCanceledByBusiness 并附带清晰提示，避免误报为 "rpc timeout"
3. Wait() / waitNoTimeout() 统一走 handleParentDone，消除分支差异

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.71%. Comparing base (bfa27f3) to head (4c2b01d).
⚠️ Report is 46 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1949      +/-   ##
==========================================
+ Coverage   61.37%   62.71%   +1.33%     
==========================================
  Files         388      393       +5     
  Lines       35063    30113    -4950     
==========================================
- Hits        21521    18885    -2636     
+ Misses      12247     9933    -2314     
  Partials     1295     1295              

Flag	Coverage Δ
integration	51.66% <0.00%> (+1.15%)	⬆️
unit	53.09% <100.00%> (+1.45%)	⬆️

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