Fix create_delta_data_intervals being ignored for timedelta schedules

[sharetheknowledge]

Fixes a bug where create_delta_data_intervals had no effect on DAGs with a
timedelta/relativedelta schedule.

_create_timetable() in task-sdk/src/airflow/sdk/definitions/dag.py checked
create_cron_data_intervals for both the cron-string branch and the
timedelta/relativedelta branch. As a result:

• create_delta_data_intervals had no effect at all — it's referenced nowhere
  else in the codebase besides CLI config-list metadata and docs.
• create_cron_data_intervals silently controlled timetable selection for
  timedelta/relativedelta schedules too, which isn't what it's documented
  to do (config.yml describes it as governing only cron-string schedules).

The timedelta | relativedelta branch now checks create_delta_data_intervals
instead, matching config.yml's documented behavior and making the two config
keys independent, as intended.

Added test_timedelta_schedule_respects_create_delta_data_intervals_config in
task-sdk/tests/task_sdk/definitions/test_dag.py, covering:

• default (both False) → DeltaTriggerTimetable
• create_delta_data_intervals=True → DeltaDataIntervalTimetable
• create_cron_data_intervals=True alone → still DeltaTriggerTimetable
  (regression guard against the exact bug fixed here)

Confirmed the new test fails against the old code and passes against the fix.
Full test_dag.py suite (94 tests) still passes.

Note: unit_tests.cfg sets both create_cron_data_intervals and
create_delta_data_intervals to true, which is why the existing test suite
never caught this — both keys being true produced the same (accidentally
correct) result before and after this fix for any test not overriding them
individually.

• closes: create_delta_data_intervals is never read - timedelta/relativedelta schedules check create_cron_data_intervals instead #69868

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