fix(ai): capture pre-calculated cost from OpenAI Agents responses

[fuchengwarrenzhu]

💡 Motivation and Context

Captures an SDK-provided response.usage.cost for OpenAI Agents response spans as $ai_total_cost_usd. This preserves gateway-calculated pricing when an OpenAI-compatible gateway routes an alias to a model whose provider differs from the apparent model.

💚 How did you test it?

• uv run pytest posthog/test/ai/openai_agents/test_processor.py -q
• uv run ruff format --check posthog/ai/openai_agents/processor.py posthog/test/ai/openai_agents/test_processor.py
• uv run ruff check posthog/ai/openai_agents/processor.py posthog/test/ai/openai_agents/test_processor.py
• uv run python -W error -c "import posthog"

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

Codex verified the targeted implementation and focused test coverage. The change only forwards a non-None pre-calculated cost, preserving explicit zero and leaving events without a cost unchanged.

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Created with PostHog Code from an inbox report.

[fuchengwarrenzhu]

This is ready for review.