fix(celery): coerce non-string values in CeleryGetter.get()

[DrMeers]

Description

CeleryGetter.get() can return non-string values (e.g. int) from Celery task request attributes, which causes TraceState.from_header() to crash with TypeError: expected string or bytes-like object, got 'int'.

Celery's Context copies all message properties as instance attributes via __dict__.update(), including non-string values like timelimit (tuple of ints) and priority (int). The TextMapPropagator contract requires string values from Getter.get(), but CeleryGetter returned these non-string values as-is.

This patch coerces all non-string values to strings before returning them.

Fixes #4359

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added unit tests for int values, iterables with non-string elements, and mixed-type iterables
• Updated existing test_get_iter assertion (now returns tuple, consistent with coercion)
• Verified fix resolves the TypeError in a production Celery worker with OTel tracing

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: DrMeers / name: Simon Meers (1d21f9b, a77f25e, fa21efa)
• ✅ login: xrmx / name: Riccardo Magliocchetti (4676f24)

[github-actions]

This PR has been automatically marked as stale because it has not had any activity for 14 days. It will be closed if no further activity occurs within 14 days of this comment.
If you're still working on this, please add a comment or push new commits.

[DrMeers]

This PR has been automatically marked as stale because it has not had any activity for 14 days. It will be closed if no further activity occurs within 14 days of this comment. If you're still working on this, please add a comment or push new commits.

Just waiting on it getting merged!