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