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.

Tests

test_timedelta_schedule_respects_create_delta_data_intervals_config in
task-sdk/tests/task_sdk/definitions/test_dag.py is parametrized over
(delta, cron, expected) and over the schedule type:

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

each run against timedelta(days=1) and relativedelta(days=1), for six
independent cases.

Verified locally: all six pass against the fix, and four of the six fail
against the pre-fix code (both delta-true rows and both cron-true rows,
including the relativedelta ones). Full test_dag.py suite: 99 passed.

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.

Docs and newsfragment

The fix changes behavior for anyone who set create_cron_data_intervals = True
on the upgrade guide's advice, so this PR documents it in four places:

• airflow-core/newsfragments/69869.significant.rst - who is affected and the
  remedy.
• upgrading_to_airflow3.rst - a bullet next to the existing
  create_cron_data_intervals one.
• timetable.rst - create_delta_data_intervals added to "Switching between
  trigger and data interval timetables".
• config.yml - the skip-one-period paragraph the cron entry already had but
  the delta entry lacked.

Setting create_delta_data_intervals = True is a no-op before this fix, so it
can be set ahead of upgrading to keep current behavior.

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

---

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Generated-by: Claude Code following the guidelines

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments.

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[sharetheknowledge]

All three review comments are addressed:

• Newsfragment plus the three doc locations (upgrading_to_airflow3.rst,
  timetable.rst, config.yml)
• conf_vars hoisted to module level
• The test is parametrized over (delta, cron, expected) and over the schedule
  type, so the three config combinations each run against timedelta and
  relativedelta