[Feat][Router] add ability to specify params to drop

[max-wittig]

Some LLMs don't support all params that clients could send. This MR gives the administrator the ability to filter those out

Similar to https://docs.litellm.ai/docs/completion/drop_params

Tested using

python3 -m vllm_router.app --port "$1" \
    --service-discovery static \
    --static-backends "http://localhost:11434" \
    --static-models "qwen3" \
    --static-model-types "chat" \
    --drop-params "prompt" \
    --routing-logic roundrobin

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[max-wittig]

@YuhanLiu11 WDYT?

[YuhanLiu11]

@max-wittig This would be nice to have. Can you write some tests for this feature?

[max-wittig]

@YuhanLiu11 I wanted to add some unittests, but the whole route_general_request has no tests at the moment.

Is there any place where you could point me to where to add them?

[YuhanLiu11]

@YuhanLiu11 I wanted to add some unittests, but the whole route_general_request has no tests at the moment.

Is there any place where you could point me to where to add them?

You can add the tests in https://github.com/vllm-project/production-stack/blob/main/tests/e2e/test-routing.py to send a request that specify params to drop and see if it did drop params from the request.

[ikaadil]

Could you please remove this comment? It is not relevant now.