[BugFix][KVCache] Fix mm hash boundary comparison in get_block_hash_extra_keys

[kevincheng2]

Motivation

修复 get_block_hash_extra_keys 中多模态图像位置与 block 边界比较的 off-by-one 错误。当图像恰好与 block 相接（image_end == block_start）时，原代码使用 < 判断导致图像被误判为与 block 有重叠，影响 prefix cache 的 hash 计算正确性。

Modifications

• 将 image_offset + image_length < start_idx 改为 <=（2处：快速返回检查和循环内跳过检查）
• 新增边界条件单元测试，覆盖图像与 block 的各种位置关系

Usage or Command

无需额外配置，修复自动生效。

Checklist

• Add at least a tag in the PR title.
• Format your code, run pre-commit before commit.
• Add unit tests.
• Provide accuracy results.

🤖 Generated with Claude Code

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

chengyanfu seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (develop@dd93f8f). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #6929   +/-   ##
==========================================
  Coverage           ?   73.25%           
==========================================
  Files              ?      399           
  Lines              ?    55608           
  Branches           ?     8764           
==========================================
  Hits               ?    40736           
  Misses             ?    11955           
  Partials           ?     2917           

Flag	Coverage Δ
GPU	73.25% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.