test(openai-agents): Set streaming header when library uses with_streaming_response()

[alexander-alderman-webb]

Description

Addresses test failures in #5574

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

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Semver Impact of This PR

🟢 Patch (bug fixes)

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Internal Changes 🔧

Openai Agents

• Set streaming header when library uses with_streaming_response() by alexander-alderman-webb in #5583

• Replace mocks with library types for streamed responses by alexander-alderman-webb in #5580

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Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 7.17s

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 13794 uncovered lines.

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