Avoid parsing templated DateTimeSensorAsync targets at Dag parse time

[SIBTAIN-ASAD]

Why

DateTimeSensorAsync can be configured with start_from_trigger=True, but templated target_time values are rendered after Dag parsing. Calling _moment in __init__ therefore parses the raw Jinja expression and crashes the Dag.

What changed

Keep triggerer-start for concrete targets, but fall back to the normal worker path when target_time contains a Jinja template. The worker path resolves the rendered datetime before constructing DateTimeTrigger.

Fixes #70284

Verification

• uv run --project providers/standard pytest providers/standard/tests/unit/standard/sensors/test_date_time.py -xvs (16 passed)
• uvx prek run --stage pre-commit --files providers/standard/src/airflow/providers/standard/sensors/date_time.py providers/standard/tests/unit/standard/sensors/test_date_time.py

[boring-cyborg]

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[potiuk]

The guard correctly stops DateTimeSensorAsync.__init__ from parsing a Jinja target_time at Dag-parse time and falls back to the worker path, where execute() resolves the rendered datetime before constructing DateTimeTrigger. That is in line with the earlier decision that the triggerer must not render templates itself, and the regression test does fail without the fix.

I pushed a small fixup on top and rebased the branch on main:

• :param start_from_trigger: now says it is ignored when target_time is a Jinja template. Without that, a Dag author who asked for triggerer-start has no way to find out why their task went through a worker.
• The test also asserts op.start_trigger_args is DateTimeSensorAsync.start_trigger_args, pinning the other half of the guard — the class attribute must be left alone rather than replaced with a copy built from the raw template. That complements test_start_trigger_args_are_not_shared_between_tasks.

Smaller observations

• The hard-coded {{ / {% / {# check will not recognise custom delimiters set through DAG(jinja_environment_kwargs=...). That matches how core already detects templates in airflow.utils.helpers.parse_template_string, so it is fine as is — just noting the parse-time crash still exists for that rare configuration.
• Mapped tasks built with partial(start_from_trigger=True).expand(target_time=[...]) skip __init__, so the guard does not run on that path. That is pre-existing and documented, not something this PR regresses.
• The PR description does not include the template's "Was generative AI tooling used to co-author this PR?" section. Could you restore it and answer it either way? See contributing-docs/05_pull_requests.rst.

---

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.