[Cherry-Pick] [BugFix] fix prefix tree updating timeout (#6615)

[liyonghua0910]

Motivation

修复 RL 场景下 prefix tree 更新（UPDATING）时因等待 inflight cache task 完成导致超时/阻塞的问题

Modifications

将 PrefixCacheManager.reset() 方法新增 wait_for_tasks_done 参数，区分 CLEARING 和 UPDATING 两种场景的等待行为

Usage or Command

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 6.25000% with 15 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/2.4@7d76fd4). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/cache_manager/prefix_cache_manager.py	6.25%	15 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release/2.4    #6617   +/-   ##
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  Coverage               ?   56.29%           
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  Files                  ?      333           
  Lines                  ?    42365           
  Branches               ?     6448           
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  Hits                   ?    23851           
  Misses                 ?    16644           
  Partials               ?     1870           

Flag	Coverage Δ
GPU	56.29% <6.25%> (?)

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