[BugFix] fix cache transfer tasks failure after cache cleared

[liyonghua0910]

Motivation

为 CacheTransferManager 引入 pause/resume 机制，由 kv_cache_status_signal 触发 pause：

• pause 时，会等待目前已经提交线程池的任务执行完毕， pause 后才开始清除缓存；
• pause 后，do_data_transfer 线程不再从上游拉取任务，已经拉取的任务本次仍可继续提交；
• resume 后，do_data_transfer 线程可以继续提交任务

Modifications

File	Modification
fastdeploy/cache_manager/cache_transfer_manager.py	新增 pause, resume 方法，新增 submit_task 用于维护 inflight 任务计数
fastdeploy/cache_manager/prefix_cache_manager.py	取消 clear prefix cache 前阻塞等待任务结果的限制
fastdeploy/inter_communicator/engine_cache_queue.py	支持清空 engine_cache_queue 队列

Usage or Command

无

Accuracy Tests

无

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.
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[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 24.00000% with 76 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@85db063). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/cache_manager/cache_transfer_manager.py	13.20%	46 Missing ⚠️
fastdeploy/entrypoints/engine_client.py	11.11%	16 Missing ⚠️
...astdeploy/inter_communicator/engine_cache_queue.py	54.16%	11 Missing ⚠️
fastdeploy/engine/common_engine.py	0.00%	2 Missing ⚠️
fastdeploy/cache_manager/prefix_cache_manager.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #6202   +/-   ##
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  Coverage           ?   66.48%           
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  Files              ?      385           
  Lines              ?    51329           
  Branches           ?     8010           
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  Hits               ?    34128           
  Misses             ?    14718           
  Partials           ?     2483           

Flag	Coverage Δ
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