fix: llm-proxy response_cost being 0

[xingyaoww]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

Fix the issue where Litellm proxy cost wasn't correctly calculated: response_cost by default is 0 but not None (a bug from the LiteLLM side), which causes our cost calculation to mess up.

This PR fixes it by only looking at 'llm_provider-x-litellm-response-cost'

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:0852fed-nikolaik   --name openhands-app-0852fed   docker.all-hands.dev/all-hands-ai/openhands:0852fed

[mamoodi]

Would be nice if enyst can take a look but looks reasonable.

[enyst]

I'm good with the cost PR, please note that this branch also includes the revert that is subject to #6300

I think it would be good to keep them separate?