sanitize OpenAI tracing export payloads

[alfozan]

Summary

Updates the OpenAI tracing exporter to align usage metadata with the trace ingest payload shape.

For the default OpenAI tracing endpoint, usage metadata is now only included on generation spans, where it is normalized to the supported token-count shape. Other tracing endpoints continue to receive the raw SDK payload.

Adds regression coverage for OpenAI endpoint sanitization and custom endpoint behavior.

Test plan

• make format
• make lint
• make sync && make typecheck
• uv run pytest tests/test_openai_responses.py::test_get_response_span_exports_usage tests/test_trace_processor.py (46 passed)
• Manual tracing repro: Runner.run(...) completed, flush_traces() completed, and no tracing client errors were emitted.
• make tests reached 3858 passed, 10 failed locally in sandbox tests because sandbox-exec returned sandbox_apply: Operation not permitted.

Issue number

N/A

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

@codex review now

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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