Reset C128 state on req slot allocation

[zhangxiaolei123456]

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CI States

Latest PR Test (Base): ❌ Run #28155292578
Latest PR Test (Extra): ❌ Run #28155292483

[gemini-code-assist]

Code Review

This pull request refactors the clearing of request-scoped C128 state for DeepSeek-V4. The clearing logic is moved from radix-cache prefix matching to request slot allocation, ensuring stale C128 state is cleared when a request slot is newly allocated while preserving state for reused slots (such as in chunked continuation). Additionally, the memory pool is updated to support batch clearing of states, and unit tests are updated to reflect these changes. I have no feedback to provide as there are no review comments.

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