[Performance]: Improve Prefix Cache Hit Rate and Reduce Dirty Cache Impact

[CLFutureX]

issue

Test Plan

Test Result

---

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]

Warning

Gemini encountered an error creating the review. You can try again by commenting /gemini review.

[CLFutureX]

Warning

Gemini encountered an error creating the review. You can try again by commenting /gemini review.

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a delayed block freeing mechanism designed to improve prefix cache hit rates. Instead of freeing blocks immediately, they are collected and freed in batches. The freeing logic is designed to prioritize the eviction of blocks from longer sequences, which should help retain shorter, more common prefixes in the cache. While the overall approach and the new test case are sound, I've identified a critical bug in the implementation that will cause a runtime error.