[Bug fix][Cherry-pick] Limit multi-modal request for prefill batch to 1(#5901)

[rainyfly]

Motivation

限制PD 分离下每次进入引擎做 prefill 的多模请求只包含 1 条。

Modifications

None

Usage or Command

None

Accuracy Tests

None

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 0% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/online/20251131@43dc335). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/sched/resource_manager_v1.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             release/online/20251131    #5902   +/-   ##
==========================================================
  Coverage                           ?   58.49%           
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  Files                              ?      320           
  Lines                              ?    39184           
  Branches                           ?     5910           
==========================================================
  Hits                               ?    22919           
  Misses                             ?    14430           
  Partials                           ?     1835           

Flag	Coverage Δ
GPU	58.49% <0.00%> (?)

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