[BugFix] fix prefix_cache_status_signal usage typo

[liyonghua0910]

Motivation

修复 prefix_cache_status_signal 的使用语法错误，该 bug 会导致推理状态下部分异常报错无法被拦截，不影响正常功能。

Modifications

Usage or Command

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
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• Add unit tests. Please write the reason in this PR if no unit tests.
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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/2.4@5ed439c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/cache_manager/prefix_cache_manager.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release/2.4    #6378   +/-   ##
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  Coverage               ?   55.61%           
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  Files                  ?      333           
  Lines                  ?    42236           
  Branches               ?     6427           
==============================================
  Hits                   ?    23488           
  Misses                 ?    16873           
  Partials               ?     1875           

Flag	Coverage Δ
GPU	55.61% <0.00%> (?)

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[paddle-bot]

Thanks for your contribution!