[BugFix] Rebuild InputBatch when max_model_len is auto-reduced

[linshi-w]

When the engine auto-reduces max_model_len to fit GPU memory after
InputBatch creation, the MLA indexer's expanded_block_table_buffer
gets fewer columns than InputBatch.block_table, causing a shape
mismatch during decode.

may_reinitialize_input_batch() only guarded against block_size
changes, which never fire for FlashMLA (block_size always 64).
Add a max_model_len comparison so the InputBatch is rebuilt.

Fixes #46787

[claude]

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[mergify]

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

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

[linshi-w]

Hi @njhill, I've rebased this PR onto the latest main — merge
conflicts and DCO are now resolved. Could you take a look and add
the ready label when you have a chance? Thanks!

[mergify]

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

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