test(openai-agents): Replace mocks with httpx in API error test

[alexander-alderman-webb]

Description

Replace mocks with httpx types to avoid test failures when library internals change.

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

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Bug Fixes 🐛

• (celery) Propagate user-set headers by sentrivana in #5581
• (utils) Avoid double serialization of strings in safe_serialize by ericapisani in #5587

Documentation 📚

• (openai-agents) Remove inapplicable comment by alexander-alderman-webb in #5495
• Add AGENTS.md by sentrivana in #5579
• Add set_attribute example to changelog by sentrivana in #5578

Internal Changes 🔧

Openai Agents

• Replace mocks with httpx in API error test by alexander-alderman-webb in #5601

• Expect namespace tool field for new openai versions by alexander-alderman-webb in #5599

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