fix(gateway): attribute inference calls to traces

[jdchawla29]

What

• Attach the ambient SDK trace ID to each OpenAI, Anthropic, and Gemini gateway request.
• Resolve trace context per request so cached clients remain safe across concurrent rollouts.
• Keep response automation's instrumented classifier span and synthetic user-step behavior, while including the classifier's gateway cost in trace and job usage.

Why

Provider clients bypass the SDK's normal request helpers, so gateway inference logs were created without trace_id. The platform could charge the owning team, but trace and job usage could not attribute those costs.

The request hook uses the existing SDK trace context rather than requiring callers to manage a separate header contract.

Validation

• Ruff check and format
• Strict pyright
• Focused gateway tests, including an actual async Gemini provider request: 4 passed
• Broader SDK utility and agent tests: 174 passed

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