Fix ExternalTaskSensor log timezone

[reachsrinivaschennupati-tech]

Make ExternalTaskSensor poke logs display the external logical date in Airflow's configured timezone.

The stored external_dates_filter value is left unchanged so listeners and integrations continue to receive the original serialized logical date.

closes: #69657

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Generated-by: Codex following the guidelines

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Local checks:

• git diff --check
• .venv/bin/ruff check --no-cache providers/standard/src/airflow/providers/standard/sensors/external_task.py providers/standard/tests/unit/standard/sensors/test_external_task_sensor.py
• .venv/bin/ruff format --check --no-cache providers/standard/src/airflow/providers/standard/sensors/external_task.py providers/standard/tests/unit/standard/sensors/test_external_task_sensor.py

I attempted the focused pytest command below, but pytest did not collect tests because the Airflow pytest plugin invokes a nested uv run and uv failed cache initialization in this local environment:

• uv run pytest providers/standard/tests/unit/standard/sensors/test_external_task_sensor.py -k "external_dag_sensor_log"

[boring-cyborg]

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