Trim weak platform block-size tests (#613)

This PR is:

- To remove platform block-size tests that only assert declarations or
no-op state.
- To tighten non-hybrid coverage so it proves delegation to vLLM's base
block-size update.
- To keep Metal block-size runtime behavior unchanged.

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Signed-off-by: Yuan Lik Xun <lxyuan0420@gmail.com>