Summary by cubic

Adds operation.cost to Anthropic spans when genai-prices is available, enabling per-request cost tracking. Tests are updated and skip on older Pydantic to avoid import issues.

• New Features
  • Compute cost from usage and model via genai-prices and set operation.cost on spans.
  • Graceful fallback if genai-prices is not installed or errors.
  • Tests assert operation.cost (and schema) across sync, async, and tool messages, and skip when Pydantic < 2.5.

^{Written for commit b99b337. Summary will update on new commits.}