ExternalTaskSensor poke log shows logical_date in UTC despite configured timezone

### Under which category would you file this issue? Airflow Core ### Apache Airflow version 3.1.8 ### What happened and how to reproduce it? When `ExternalTaskSensor` logs its poke message, the datetime shown after `on` is serialized directly with `datetime.isoformat()`. ```shell Poking for DAG 'example_external_dag' on 2026-07-06T21:00:00+00:00 ... ``` In deployments configured with a non-UTC timezone, such as AIRFLOW__CORE__DEFAULT_TIMEZONE=Asia/Seoul, this can be confusing because task log timestamps may be shown in the configured/local timezone while the ExternalTaskSensor poke message still displays the external logical date in UTC. **Current code path** ```python def _serialize_dttm_filter(dttm_filter): return ",".join(dt.isoformat() for dt in dttm_filter) ``` **Steps to reproduce** Configure Airflow with a non-UTC default timezone, for example Asia/Seoul. Create a Dag with an ExternalTaskSensor waiting for another Dag. Run the sensor task. Observe the poke log message. ### What you think should happen instead? The poke log should make the timezone easier to understand, ideally by formatting the datetime consistently with Airflow's configured timezone or by otherwise making clear that the value is the external Dag run `logical_date`. This would reduce confusion when operators compare the sensor poke log with Airflow UI timestamps or local operational time. ### Operating System _No response_ ### Deployment None ### Apache Airflow Provider(s) _No response_ ### Versions of Apache Airflow Providers _No response_ ### Official Helm Chart version Not Applicable ### Kubernetes Version _No response_ ### Helm Chart configuration _No response_ ### Docker Image customizations _No response_ ### Anything else? _No response_ ### Are you willing to submit PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)