Use AttentionSelectorConfig in get_attn_backend_cls

[karan]

Description

vllm-project/vllm#30212 refactored individual attention parameters into AttentionSelectorConfig. That broke our platform implementation - this PR fixes that.

I also added **kwargs to the signature. IMO, we should keep that there as a safety net if vllm adds more platform-specific args to the selector call later, it won't immediately break us.

Tests

Ran a model locally:

vllm serve openai/gpt-oss-120b --max-model-len=9216 --max-num-batched-tokens=1024 --max-num-seqs=128 --no-enable-prefix-caching --gpu-memory-utilization=0.98 --tensor-parallel-size=4 --kv-cache-dtype=fp8 --async-scheduling

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[github-actions]

Description

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FIXES: #123456

Tests

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commands to reproduce.

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.