feat: enhance LiteLLM metadata extraction by adding cost retrieval from response headers

[viniciusdsmello]

Pull Request

Summary

Adds a fallback mechanism to capture cost from LiteLLM response headers when response_cost is zero or unavailable.

Changes

• Modified extract_litellm_metadata to track response headers and check for cost fallback
• Added _extract_cost_from_headers helper function to parse x-litellm-response-cost and x-litellm-response-cost-original headers

Context

In some cases, the response_cost from LiteLLM's hidden params is returned as 0, but the actual cost is available in the response headers under x-litellm-response-cost. This fix ensures we capture cost data from headers as a fallback when the primary source fails.

Applies to both streaming and non-streaming completions.

Testing

• Unit tests
• Manual testing
• Postman CI/CD
• Other (please specify)