chores: adjust the attn register param order

[ILikeIneine]

Purpose

This is a follow-up to #26487.

It accidently change the order of attention registry default parameter, which caused the failure in normal registry and crashed all the workflow of our plugins tests (the vllm-metax).

For not adding class_path = everywhere in the registry code in non-mamba cases ( and I think that's the most common situation) , it is needed to exchange the order of the default parameter.

Test Plan

By plugin

Test Result

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[gemini-code-assist]

Code Review

This pull request corrects the parameter order in the register_backend function signature. The previous order caused issues with positional arguments, particularly for direct backend registration. By swapping class_path and is_mamba, this change restores the intended behavior and improves the API's usability for the most common use cases. The change is correct and well-justified. I approve this change.

[mgoin]

Makes sense to me to be later in the arg list, thank you