fix: panic not triggered when all retries return ErrRPCFinish

[yangma0]

[What type of PR is this?](fix: panic not triggered when all retries return ErrRPCFinish)

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• 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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修复 MixedRetryer.Do 中所有重试请求返回 ErrRPCFinished 时未能触发 panic 的问题

(Optional) More detailed description for this PR(en: English/zh: Chinese).
en:
This PR adds a test case to reproduce a rare panic scenario where all retry attempts in MixedRetryer.Do return ErrRPCFinished, leading to unexpected behavior (no panic triggered and nil response returned).
A fix is also provided to ensure proper panic is raised when all responses are ErrRPCFinished, maintaining consistency with the retry strategy design.
zh(optional):
本 PR 添加了一个测试用例，用于复现 MixedRetryer.Do 方法中所有重试请求都返回 ErrRPCFinished 时未能触发预期 panic 的问题。同时也修复了该逻辑，确保在这种极端场景下能够正确 panic，防止返回预期之外的err导致程序运行异常。
(Optional) Which issue(s) this PR fixes:

(optional) The PR that updates user documentation:

[CLAassistant]

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[ppzqh]

thanks for the feedback, we will review the code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.44%. Comparing base (b3a437e) to head (3080f32).
Report is 1 commits behind head on main.

Additional details and impacted files

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##             main    #1814   +/-   ##
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  Coverage   60.44%   60.44%           
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  Files         380      380           
  Lines       34485    34486    +1     
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+ Hits        20843    20846    +3     
+ Misses      12376    12372    -4     
- Partials     1266     1268    +2     

Flag	Coverage Δ
integration	48.26% <11.11%> (-0.05%)	⬇️
unit	50.91% <100.00%> (+0.09%)	⬆️

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