[0.13.0][cherry-pick][bugfix](cp) align max_context_chunk to cp_virtual_block_size

[pisceskkk]

What this PR does / why we need it?

In the chunked prefill scenario, CP needs to align the max_context_chunk to the cp_virtual_block_size, but the current implementation only aligns it to the block_size. For PD-disaggregation, cp_kv_cache_interleave_size is typically set equal to block_size, in which case cp_virtual_block_size=block_size * dcp_size * pcp_size. Under specific conditions, this can lead to misalignment of certain chunks, subsequently triggering assertion check errors.
ref. #5767

Does this PR introduce any user-facing change?

No

• vLLM version: v0.13.0
• vLLM main: vllm-project/vllm@2f4e654

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in the chunked prefill scenario with context parallelism by ensuring max_context_chunk is properly aligned. The approach of updating self.block_size to be the least common multiple of its original value and cp_virtual_block_size is sound. I've added one suggestion to improve robustness by adding an assertion to prevent a potential division-by-zero error in case of a misconfiguration. Overall, this is a good fix.