fix prefix caching

[yao-fengchen]

No description provided.

[jinminxi104]

add a testcase to ci, for testing mixed prefill_with_kvcache and prefill_with_kvcache. (ex: chunked prefill size = 2k; 3 prompts length are 3k,0.2k, 0.2k

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✅ yao-fengchen
❌ root

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