pydantic-ai: read RequestUsage.output_reasoning_tokens for reasoning metrics #1005

Sub-issue of #1001 (follow-ups from the 2026-10-26 dependency update, #998).

• We get reasoning tokens by checking provider-specific keys in usage.details (integrations/pydantic_ai/tracing.py:1424-1440). That still works because pydantic-ai keeps filling those keys.
• The new field is provider-agnostic. Reading it first, with the keys as fallback, guards against the keys going away or providers we don't map. Covered by existing cassettes.