[Cherry-Pick][BugFix] Fix async download(#5349)

[kevincheng2]

Motivation

💡 If this PR is a Cherry Pick, the PR title needs to follow the format by adding the [Cherry-Pick] label at the very beginning and appending the original PR ID at the end. For example, [Cherry-Pick][CI] Add check trigger and logic(#5191)

💡 如若此PR是Cherry Pick，PR标题需遵循格式，在最开始加上[Cherry-Pick]标签，以及最后面加上原PR ID，例如[Cherry-Pick][CI] Add check trigger and logic(#5191)

1. 修复PD分离场景下异步下载没有生效问题
2. 取消MTP场景下关闭prefix cache的限制
3. 修复MTP+prefix cache场景下，MTP没有创建cache的问题

Cherry-Pick from #5349

Modifications

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 16.12903% with 26 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/online/20251131@f1e1f5d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	0.00%	19 Missing ⚠️
fastdeploy/engine/request.py	14.28%	6 Missing ⚠️
fastdeploy/spec_decode/mtp.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             release/online/20251131    #5347   +/-   ##
==========================================================
  Coverage                           ?   59.02%           
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  Files                              ?      319           
  Lines                              ?    38871           
  Branches                           ?     5843           
==========================================================
  Hits                               ?    22945           
  Misses                             ?    14095           
  Partials                           ?     1831           

Flag	Coverage Δ
GPU	59.02% <16.12%> (?)

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