Extract openai usage using genai-prices

[alexmojaki]

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Pull Request Overview

This PR extracts OpenAI usage information using the "genai-prices" library. The changes update the usage mapping functionality to work with genai-prices for better token cost calculation and tracking.

• Update genai-prices dependency to version 0.0.31
• Refactor _map_usage function to use RequestUsage.extract() method from genai-prices
• Add base_url parameter tracking for provider URL identification

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.

File	Description
pydantic_ai_slim/pyproject.toml	Bump genai-prices dependency from 0.0.30 to 0.0.31
pydantic_ai_slim/pydantic_ai/models/openai.py	Refactor usage mapping to use genai-prices extraction with provider URL tracking
tests/models/mock_openai.py	Add base_url field to mock OpenAI classes for testing

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