[KVConnector] Enable get_block_ids_with_load_errors() in LMCache connector

[ziruiliu]

Purpose

In LMCache engine, the PR LMCache/LMCache#1835 implement interface get_block_ids_with_load_errors which was introduced in #19330. So vLLM is able to handle failed cache retrieval from LMCache

Test Plan

See PR's comment at LMCache/LMCache#1835 (comment)
In the test, we try to inject fault in cache retrieval when cache lookup returns OK. vLLM is expected to receive the failed block ids from LMCache and reschedule the request

Test Result

Reschedule happened as expected

 [scheduler.py:1536] Recovered from KV load failure: 1 request(s) rescheduled (64 tokens affected).

---

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.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors.

You ask your reviewers to trigger select CI tests on top of fastcheck CI.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

If you have any questions, please reach out to us on Slack at https://slack.vllm.ai.

🚀

[gemini-code-assist]

Code Review

This pull request enables error handling for failed KV cache loads from LMCache by implementing the get_block_ids_with_load_errors method in the LMCacheConnectorV1. The implementation correctly provides a fallback for older versions of LMCache that do not support this feature. My review includes a suggestion to improve the robustness of the implementation by checking if the method is callable, which prevents potential TypeError exceptions if the attribute exists but is not a method. This is particularly important for ensuring stability when interacting with versioned dependencies.

[ApostaC]

LGTM!

[ziruiliu]

Hi @NickLucche, coul you please take a look on this change? This is quite straightfoward to enbale LMCache to report block ids those are failed in retrieval

[NickLucche]

Thanks for contributing @ziruiliu !