feat(sdk): type reasoning token usage

[KillerQueen-Z]

What changed

• Add typed prompt_tokens_details and completion_tokens_details fields to ChatUsage.
• Expose a convenience reasoning_tokens property.
• Document that reasoning tokens are already included in completion/output totals.
• Add unit coverage for populated and absent reasoning breakdowns.

Why

The SDK previously preserved these fields only as untyped Pydantic extras, which made downstream integrations fragile and easy to drop.

Validation

• 2 SDK unit tests passed.
• Python bytecode compilation passed.
• git diff --check passed.