bugfix: fix DeepSeek V4 schedule_overlap + mtp.

[JC-ut0]

Description

This PR fixes a DeepSeek V4 precision regression when enable_schedule_overlap=true and MTP are enabled. The root cause was that Sequence::update_last_step_token() used num_kv_blocks()==0 to decide whether to skip last-step token replacement, which incorrectly treated composite KV layouts as KV-less sequences. The fix switches the guard to current_max_tokens_capacity()==0, so preempted sequences are still protected while composite KV sequences keep their accepted MTP tokens. The patch also adds a regression test covering the composite KV case and preserves full replacement of accepted MTP tokens in overlap mode.

Related Issues

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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Reviewer Notes

[gemini-code-assist]

Code Review

This pull request updates Sequence::update_last_step_token to check current_max_tokens_capacity() instead of num_kv_blocks() when determining if a sequence's KV cache has been deallocated, accommodating composite KV managers that can have capacity without exposing local blocks. It also adds a corresponding unit test OverlapMTPReplacementKeepsCompositeKvBlocks to verify this behavior. Feedback was provided to annotate a constant argument with its parameter name in the new test code, in accordance with the repository style guide.

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