Fix Dag run scheduling after coarser schedule changes

[anmolxlight]

Summary

• prevent data-interval timetables from re-emitting an existing logical date after a Dag schedule changes to a coarser cadence
• keep the existing zero-length interval guard working while covering hourly-to-daily schedule changes
• add a regression test for the issue scenario

Fixes #66754

Tests

• uv run ruff format airflow-core/src/airflow/timetables/interval.py airflow-core/tests/unit/timetables/test_interval_timetable.py
• uv run ruff check --fix airflow-core/src/airflow/timetables/interval.py airflow-core/tests/unit/timetables/test_interval_timetable.py
• uv run --project airflow-core pytest airflow-core/tests/unit/timetables/test_interval_timetable.py -q

[ashb]

This change seems too simple, it'll need some careful thinking about to make sure there aren't any edge cases