[CI] Fix test_shared_storage_connector_hashes

[chaunceyjiang]

Purpose

FIX #25745

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request aims to fix a CI issue. It introduces a pytest fixture to ensure KV transfer groups are shut down after tests, preventing state leakage between modules. However, it also disables a test case for the 'ray' backend and hardcodes a user-specific model path, which will cause issues. My review focuses on making the test suite more robust and portable by addressing these two points.

[NickLucche]

Awesome job spotting this @chaunceyjiang !

Also tbh I don't have a strong opinion on this invariant https://github.com/vllm-project/vllm/blob/main/vllm/distributed/kv_transfer/kv_connector/v1/metrics.py#L54, it was just a quick way to prevent misuse.
We could also remove that line if this leads to unclear tests behavior or even worse if there are cases where VLLM_ENABLE_V1_MULTIPROCESSING=0 can be broken in a real scenario.. though I believe that should not be the case.

[DarkLight1337]

Merging to unblock CI