[Bugfix] Zero recycled KV cache blocks for FullAttention models

[AjAnubolu]

Summary

Closes #39146. The KV block zeroing pipeline from #35219 was gated to Mamba-only models; enabling it for FullAttention prevents stale K/V in partial-block tail slots from propagating NaN through masked softmax.

[gemini-code-assist]

Code Review

This pull request updates the needs_kv_cache_zeroing property to include models using FullAttentionSpec, preventing stale K/V data leakage in partial-block tail slots as identified in issue #39146. A regression test was added to verify this behavior. Feedback suggests using isinstance() for type checking to ensure compatibility with subclasses like MLAAttentionSpec and to follow PEP 8 guidelines.

[gemini-code-assist]

Using type(g.kv_cache_spec) is FullAttentionSpec is overly restrictive as it excludes subclasses like MLAAttentionSpec and SinkFullAttentionSpec. These variants of full attention likely suffer from the same stale K/V issues in partial blocks and should also benefit from zeroing. Following PEP 8 recommendations, object type comparisons should use isinstance() instead of comparing types directly, which also ensures consistency with the has_mamba_layers implementation.

Suggested change

	return self.has_mamba_layers or any(
	type(g.kv_cache_spec) is FullAttentionSpec for g in self.kv_cache_groups
	)
	return self.has_mamba_layers or any(
	isinstance(g.kv_cache_spec, FullAttentionSpec) for g in self.kv_cache_groups
	)

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @AjAnubolu.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @AjAnubolu.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork