[BugFix][Cherry Pick] fix ds type bug

[kevincheng2]

Motivation

将ds下发的mm_positions dict类型转换为ImagePosition 类型

Cherry-Pick from #5182

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!

[Copilot]

Pull request overview

This PR fixes a bug where mm_positions received from the disaggregation scheduler (ds) is in dict format instead of ImagePosition objects. The fix adds conversion logic in Request.from_dict() to transform dict entries to ImagePosition instances.

Key Changes

• Added type conversion for mm_positions from dict to ImagePosition objects in the deserialization process
• Added error handling with traceback logging for conversion failures
• Removed unused fastdeploy.envs import

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/online/20251131@a11d17c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/request.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             release/online/20251131    #5220   +/-   ##
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  Coverage                           ?   59.11%           
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  Lines                              ?    38688           
  Branches                           ?     5815           
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  Hits                               ?    22871           
  Misses                             ?    13992           
  Partials                           ?     1825           

Flag	Coverage Δ
GPU	59.11% <71.42%> (?)

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