[BugFix] Fix inaccurate cache hit rate and TTFT after request preemption

[liyonghua0910]

Motivation

修复请求被抢占后缓存命中率和 TTFT（Time to First Token）日志打印不准确的问题。

Modifications

• Request 类新增 has_been_preempted_before 标志，表示请求是否有被抢占过（发生重调度）
• RequestMetrics 类新增 preempted_count 计数，表示请求被抢占的次数
• 仅对首次被调度的请求记录 INFERENCE_START 事件，重调度的请求记为 RESCHEDULE_INFERENCE_START 事件
• 只对首次被调度的请求记录缓存命中率指标，重调度的请求的缓存命中情况不作为请求最终指标

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 48.64865% with 19 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@fa21fd9). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	0.00%	18 Missing ⚠️
fastdeploy/engine/sched/resource_manager_v1.py	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #6620   +/-   ##
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  Coverage           ?   71.61%           
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  Files              ?      394           
  Lines              ?    53963           
  Branches           ?     8469           
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  Hits               ?    38647           
  Misses             ?    12532           
  Partials           ?     2784           

Flag	Coverage Δ
GPU	71.61% <48.64%> (?)

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