tests: skip test_celery gracefully when celery/testcontainers aren't installed

[dchaudhari7177]

Fixes #750.

test_celery generates a test module that does from celery import Celery / from celery.contrib.testing import worker / from testcontainers.redis import RedisContainer. When those packages aren't installed the inner pytester.runpytest(...) errors out with ImportError, so the test fails hard instead of being skipped. The existing sys.platform skips only cover Windows/macOS, so on other platforms (e.g. FreeBSD, per the report) it blows up.

This adds a skipif guarding on celery and testcontainers being importable (via importlib.util.find_spec), matching how the suite already gates optional/platform-specific tests. When both are installed the test runs exactly as before.

Verified locally: with neither package installed, test_celery now reports SKIPPED [celery and testcontainers are required] instead of erroring; ruff check/format clean.

Happy to add a CHANGELOG.rst entry in whatever section you'd prefer for an unreleased test-only fix.