[None][feat] Integrate KVCMv2 commit_min_snapshot in runtime

[jiaganc]

@coderabbitai summary

Description

Integrate commit_min_snapshot support for SSM reuse from #15752 and connect it to the KV cache manager V2 block-reuse policies.

The cache manager now commits a minimal SSM snapshot when block reuse is enabled for policies other than all_reusable, propagates partial-reuse configuration, and handles the final context commit path. The change includes coverage for SSM/SWA reuse, partial-page commits, cache statistics, DeepSeek V4 sparse cache configuration, and V2 reuse-policy selection.

Test Coverage

• Pre-commit hooks passed for both commits.
• python3 -m py_compile passed for the directly modified Python implementation and test files.
• Targeted pytest files were not run locally because the current environment does not have pytest installed.

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[jiaganc]

This PR depends on #15752 and should wait for it to be merged first.