Refine shared DCP decode metadata

[GirasoleY]

Follow-up to vllm-project#52188. This intentionally targets the existing PR source branch rather than opening a duplicate upstream PR.

What changed:

• Move flattened decode metadata onto MLACommonDecodeMetadata.
• Remove the FlashInfer-only metadata class hierarchy while preserving causal FlashInfer target verification.
• Reuse the same flattened sequence lengths and query boundaries in generic and Kimi-K3 DCP combine.
• Keep the optional FlashInfer test import inside its SM100-gated test.

CUDA graphs:
The existing capture path records the one-time flattening operations with graph-owned outputs and stable replay addresses. This follow-up therefore does not add separate persistent builder buffers.

Validation:

• uvx --offline ruff check on all changed files: passed.
• uvx --offline ruff format --check on all changed files: passed.
• Python compileall on all changed files: passed.
• git diff --check: passed.
• GPU tests and model evaluations were not run locally; no compatible GPU environment was available.

AI assistance:
OpenAI Codex was used to inspect the existing PR, prepare the patch, and run the listed checks. The change is published for human review before merging.

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