Add operation.cost span attribute to model request spans, rename ModelResponse.price() to .cost()

[alexmojaki]

This gives the cost of a request in USD as calculated by genai-prices, if possible.

ModelResponse.price() is now deprecated in favour of ModelResponse.cost() which does the same thing.

[github-actions]

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[Copilot]

Pull Request Overview

This PR adds support for tracking operation costs in OpenTelemetry spans by including an operation.cost attribute when pricing information is available for AI model requests.

• Adds operation.cost span attribute to track the financial cost of AI model operations
• Updates test fixtures to use realistic OpenAI models with known pricing (gpt-4o variants)
• Handles pricing lookup errors gracefully by omitting the cost attribute when pricing is unavailable

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pydantic_ai_slim/pydantic_ai/models/instrumented.py	Adds cost tracking logic with error handling for pricing lookups
tests/models/test_instrumented.py	Updates test model names and adds expected cost attributes to test assertions

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