fix(celery): type getter and harden utility helpers

[diogosilva30]

Description

Split out of #4439.

This PR keeps only the Celery housekeeping changes:

• type CeleryGetter and normalize its returned values
• add null guards and helper typing in utils.py
• refine utility helper protocols and context aliases to match actual call sites
• update getter tests to match the typed behavior
• add a Sphinx nitpick exception for celery.worker.request.Request used in the documented getter type annotation

Testing

• uv run tox -e py311-test-instrumentation-celery -- -q
• uv run tox -e docs -- -W

[MikeGoldsmith]

Thanks @diogosilva30 🚀

[diogosilva30]

@MikeGoldsmith rebased from main, and added towncrier fragment instead of changelog edit

[emdneto]

Thanks for the PR!

Just a heads-up: we no longer update CHANGELOG.md directly. The changelog is now generated from changelog fragments using Towncrier.

Please add the appropriate changelog fragment for this change instead of editing CHANGELOG.md manually. You can find the instructions and expected format in CONTRIBUTING.md.