[Router][Bugfix] Select longest LMCache prefix match

[dsxyy]

Summary

• select the LMCache layout_info entry with the largest matched_prefix_length
• use that same instance for the KV-aware threshold check and backend selection
• add a regression test covering both direct misrouting and incorrect fallback when a later entry is the longest match

LMCache defines layout_info as instance_id -> (location, matched_prefix_length), but does not define dictionary order as a best-match ordering. Its controller builds the mapping while walking chunks, so the first inserted instance is not necessarily the longest match.

Tests

57 passed in 3.27s

The regression test fails twice on current main and passes with this change.

Static checks run on the changed files:

• black 25.1.0
• isort 6.0.0
• ruff 0.12.0
• codespell 2.4.1
• git diff --check

All passed.

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[gemini-code-assist]

Code Review

This pull request updates the KV-cache-aware routing logic to route requests to the instance with the longest matched prefix (maximum matched tokens) instead of simply picking the first instance in the layout info, and adds corresponding unit tests. The review feedback identifies a potential KeyError if the matched instance ID is not found in the active endpoints mapping, suggesting a fallback mechanism to session or QPS routing. Additionally, it recommends stubbing QueryInstMsg in the test suite to prevent potential NameError exceptions when lmcache is not installed.

[chatgpt-codex-connector]

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[ruizhang0101]

LGTM

[dsxyy]

@ruizhang0101 Thanks for the approval! All checks are green now. Could you help merge this?