ref(celery): Remove unused NoOpMgr from utils

[sentrivana]

In the Celery integration, we use a custom NoOpMgr to make working with context managers a bit easier. It's defined in the file where we use it. There is another, completely unused copy in the utils.py of the Celery integration. Remove that one.

Note: Once we drop 3.6 support, we can replace the custom class with nullcontext from the standard library.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Entries from this PR are highlighted with a left border (blockquote style).

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New Features ✨

• (ci) Cancel in-progress PR workflows on new commit push by joshuarli in #5994

Bug Fixes 🐛

• (google_genai) Redact binary data in inline_data and fix multi-part message extraction by ericapisani in #5977

Internal Changes 🔧

• (celery) Remove unused NoOpMgr from utils by sentrivana in #6078

• (pydantic-ai) Remove dead Model.request patch by alexander-alderman-webb in #5956
• Set explicit base-branch for codecov action by ericapisani in #5992

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Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 9.22s

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 15028 uncovered lines.

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