[Bugfix] Pick KV cache block size from all attention backends

[jayzuccarelli]

Purpose

Fixes #48286.

Platform.update_block_size_for_backend() only consulted the first non-SSM
attention backend when auto-picking the KV cache block size. Models that mix a
permissive main backend with a stricter secondary one — e.g. DSA models whose
DeepseekV32IndexerBackend only supports block size 64 — kept the default of
16 and crashed later in select_common_block_size():

ValueError: No common block size for 16.

This change collects all distinct non-SSM backends and takes the largest
preferred block size, so the indexer's requirement is honored no matter which
layer registers first. Not a duplicate: no open or closed PR addresses #48286
(checked issue timeline and recent PRs).

AI assistance was used for this PR; the change was reviewed and tests were run before submitting.

Test Plan

pytest tests/v1/worker/test_gpu_model_runner.py::test_update_block_size_for_backend_honors_strictest_backend

The new test fails on main (assert 16 == 64) and passes with this change.
Also verified against the real backend on CPU:

select_common_block_size(16, [DeepseekV32IndexerBackend])  # reproduces the reported ValueError
Platform.update_block_size_for_backend(cfg)                # with an indexer layer present
cfg.cache_config.block_size                                # -> 64

No GPU available locally, so no e2e serving run of a DSA model; the sibling
select_common_block_size unit tests and pre-commit run (ruff, mypy) pass.

Test Result

$ pytest tests/v1/worker/test_gpu_model_runner.py -k block_size -q
5 passed, 30 deselected in 1.97s

(4 pre-existing block-size tests plus the new one.)

[claude]

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[jayzuccarelli]

Friendly ping — this fixes a real crash path when hybrid attention backends disagree on supported block sizes (repro in the description). Could a maintainer add the ready label to kick off full CI? Happy to iterate on review feedback.

[mergify]

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

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

[jayzuccarelli]

Rebased onto main, conflicts resolved. Could a maintainer add the ready label to run CI?

[mergify]

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

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, @jayzuccarelli.

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

[jayzuccarelli]

Rebased onto latest main. Conflicts are cleared and the branch is mergeable again. Could a maintainer add the ready label so CI can run? Happy to iterate on any review feedback.

[mergify]

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

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

[jayzuccarelli]

Closing in favour of #49845, which fixes #48286 the same way and merged yesterday. Thanks @divyvasal.