Pass packed boundary metadata to Qwen3.5 linear-attention fast kernels from data collator

[sdharani91]

What does this PR do?

This is a follow up to #44867

This PR fixes Qwen3.5 padding-free packed inputs on the linear-attention fast path by consuming collator-provided packed metadata. The linear-attention block now uses seq_idx for the causal convolution path and cu_seq_lens_q / cu_seq_lens_k for the FLA path, matching the repo’s existing DataCollatorWithFlattening contract. I also added a deterministic fast-path regression test comparing padded and padding-free inputs, plus a slow-path contract test that raises clearly when padding-free kwargs are passed without fast-kernel support. The slow fallback implementation itself is unchanged in this PR.

Fixes # 44717

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@vasqu

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: qwen3_5