[ISSUE-68912] feat(celery): honor json_logs config in Celery worker startup

[hanxdatadog]

Closes #68912

Problem

The Celery worker ignores [logging] json_logs / AIRFLOW__LOGGING__JSON_LOGS. It always calls configure_logging(output=sys.stdout.buffer) without passing json_output, so structured JSON logging is never enabled on the worker even when configured globally.

Solution

Add a two-level config lookup before calling configure_logging:

1. [celery] json_logs — worker-specific override, takes precedence.
2. [logging] json_logs — global setting, used when the celery key is absent.
3. False — default when neither key is configured.

This mirrors the existing [celery] CELERY_LOGGING_LEVEL / [logging] LOGGING_LEVEL fallback pattern already present in the worker startup code.

# Global — affects the API server, scheduler, and workers:
[logging]
json_logs = True

# Or override for the Celery worker only, leaving other components unchanged:
[celery]
json_logs = True

Changes

• providers/celery/src/airflow/providers/celery/cli/celery_command.py: read [celery] json_logs → [logging] json_logs → False and pass json_output to configure_logging.
• providers/celery/provider.yaml + get_provider_info.py: new json_logs config option under [celery].
• providers/celery/tests/unit/celery/cli/test_celery_command.py: TestWorkerJsonLogs — 3 tests covering (a) default is False, (b) global [logging] setting is inherited, (c) [celery] override takes precedence.
• providers/celery/docs/celery_executor.rst: new "Worker logging" section documenting the fallback order and Airflow 3.0/3.2 compatibility note.
• providers/celery/newsfragments/68912.feature.rst: changelog entry.

Notes

• Only active on Airflow 3+ (AIRFLOW_V_3_0_PLUS guard is unchanged).
• conf.getboolean(..., fallback=None) is safe on 3.0/3.1 even when the [celery] json_logs key is absent.
• version_added: ~ in provider.yaml — version not yet decided; to be set by the release manager.