fix(celery): clear task timing state after postrun

[diogosilva30]

Description

Split out of #4439.

This PR keeps the Celery fix narrowly focused on the task_id_to_start_time lifecycle:

• keep the timing map instance-scoped while preserving singleton state across repeated CeleryInstrumentor() calls
• reset it when instrumenting and uninstrumenting
• remove completed task ids in _trace_postrun
• add regression coverage that verifies completed tasks clear the timing cache

Testing

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

[MikeGoldsmith]

Thanks @diogosilva30 👍🏻

[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.