[RL] [KVCache] let cache transfer managers update key prefix after weight update and add unit tests

[liyonghua0910]

Motivation

This PR updates the mooncake key_prefix during update_weights.

Previously, key_prefix was only refreshed in resume. To make cache metadata align with the new model version earlier, this PR adds one more refresh in update_weights, while keeping the existing refresh in resume.

Modifications

• Add cache-transfer control handling in EngineService._control_update_weights()
• Add update_weights handler in CacheTransferManager
• Keep resume refreshing key_prefix
• Add unit tests for both update_weights and resume paths

Usage or Command

No new API.

Behavior after this PR:

• update_weights refreshes key_prefix
• resume also refreshes key_prefix

pytest -q tests/engine/test_common_engine.py -k "control_update_weights"
pytest -q tests/cache_manager/test_cache_transfer_manager.py -k "handle_resume or handle_update_weights"

Accuracy Tests

N/A

Checklist

• Add at least a tag in the PR title.
• Format your code, run pre-commit before commit.
• Add 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 75.00000% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@a7f52c3). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
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  Coverage           ?   73.74%           
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  Files              ?      402           
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  Hits               ?    41637           
  Misses             ?    11876           
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Flag	Coverage Δ
GPU	73.74% <75.00%> (?)

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