[BugFix] fix async download bug

[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. 取消投机解码场景下关闭prefix cache的限制
3. 修复MTP+prefix cache场景下，MTP没有创建cache的问题

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 9.37500% with 29 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@b5a7abe). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	0.00%	20 Missing ⚠️
fastdeploy/engine/request.py	14.28%	6 Missing ⚠️
fastdeploy/engine/sched/resource_manager_v1.py	50.00%	2 Missing ⚠️
fastdeploy/spec_decode/mtp.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #5349   +/-   ##
==========================================
  Coverage           ?   59.22%           
==========================================
  Files              ?      325           
  Lines              ?    40239           
  Branches           ?     6096           
==========================================
  Hits               ?    23832           
  Misses             ?    14573           
  Partials           ?     1834           

Flag	Coverage Δ
GPU	59.22% <9.37%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.