Fix MoE kernel bounds checks

[yinli-systems]

Fixes #45884.
Fixes #45492.

This PR tightens two MoE CUDA kernel indexing paths:

• promotes the WNA16 output offset calculation to 64-bit before pointer indexing, avoiding overflow for large token_index * size_n + offset_n values
• validates expanded_source_row before writing expanded_source_row_to_expanded_dest_row[expanded_source_row], and skips copy work for invalid skipped rows emitted by the permute path

The root cause is that both kernels trusted index values that can exceed the valid 32-bit or expanded-row range under large or skipped MoE routing inputs.

Testing:

• git diff --check
• local boundary script covering the examples from [Bug]: Integer overflow in moe_wna16.cu #45884 and [Bug]: Out of bounds in moe_permute #45492

I do not have local CUDA hardware in this environment, so I could not run the MoE CUDA test suite locally.

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[yinli-systems]

Thanks for the bot checks. The pre-run check is blocked by the new-contributor gate, not by the patch itself: it requires either the ready/verified label or 4 merged PRs from the author.

Could a maintainer please add ready or verified when you have a chance? I kept this PR scoped to the two reported MoE indexing issues and validated the exact boundary examples from #45884 and #45492 locally with git diff --check.