router: fix lora-only route rate limits

[avinxshKD]

/kind bug

What this PR does / why we need it:

Fixes rate limits for LoRA-only ModelRoutes.

Previously the router registered the limiter only with spec.modelName. For a LoRA-only route, modelName is empty, so the limiter was added under an empty key and requests using the LoRA adapter name were not limited.

This applies the same limiter to configured loraAdapters too.

Which issue(s) this PR fixes:
None

Special notes for your reviewer:
NONE

Does this PR introduce a user-facing change?:

Fix rateLimit not being applied to LoRA-only ModelRoutes.

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[gemini-code-assist]

Code Review

This pull request updates the router to configure and delete rate limiters for LoRA adapters in addition to the base model, and adds a corresponding unit test. The reviewer identified a critical issue where unconditionally deleting rate limiters upon a ModelRoute deletion could leave other active routes sharing the same model or LoRA adapters unprotected, and suggested checking if the model or adapter is still registered in the store before deletion.

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[avinxshKD]

Hey @hzxuzhonghu @LiZhenCheng9527 added a minimal fix for LoRA-only ModelRoute rate limits.

Also covered the delete case so a limiter is not removed while another active rate-limited route still uses the same LoRA adapter. Please review when you get time.