Fix custom timetables silently failing to schedule on Airflow 3.2

[Abdulrehman-PIAIC80387]

Airflow 3.2 extended DagRunInfo with two new fields for partition-oriented scheduling (AIP-76, #61167):

class DagRunInfo(NamedTuple):
    run_after: DateTime
    data_interval: DataInterval | None
    partition_date: DateTime | None   # new in 3.2
    partition_key: str | None         # new in 3.2

Both were added without defaults, making them required positional fields. Custom timetables written for 3.1.x and earlier — including the pattern shown in the official custom-timetable how-to — construct DagRunInfo with only two arguments:

return DagRunInfo(
    run_after=run_after,
    data_interval=DataInterval(start=start, end=end),
)

On 3.2 this raises TypeError: DagRunInfo.__new__() missing 2 required positional arguments: 'partition_date' and 'partition_key'. The exception is swallowed by the error handling around DAG.next_dagrun_info(), so next_dagrun / next_dagrun_create_after stay NULL, the Next Run column is empty, and no scheduled runs are created — with no import error or other visible failure. Built-in timetables are unaffected because they build DagRunInfo via helpers (DagRunInfo.interval(...), DagRunInfo.exact(...), etc.) that already pass the partition fields explicitly.

Fix

Default the two partition fields to None. They are the trailing fields of the NamedTuple, so the change is purely additive and restores the documented two-argument construction. Every internal call site already passes all four fields explicitly, so their behaviour is unchanged.

Design notes

• Defaulting trailing NamedTuple fields is the idiomatic approach already used in this codebase — e.g. TaskInstanceKey (try_number: int = 1, map_index: int = -1). No __new__ override or dataclass conversion is needed.
• Partitioned timetables continue to set partition_date / partition_key explicitly; the defaults only affect callers that omit them, which is exactly the pre-3.2 custom-timetable contract.

Tests

Added two unit tests in test_base_timetable.py: the backward-compatible two-argument construction (partition fields default to None) and the explicit four-argument construction (partition fields still settable). Full airflow-core/tests/unit/timetables/ suite passes (220 passed).

closes: #68315

[boring-cyborg]

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[Abdulrehman-PIAIC80387]

Hi @jroachgolf84, thanks for the review!

All comments are resolved in commit 47245e5. Any further feedback is welcome!

[Lee-W]

#68342 is closer to being merged. So I'm going to merge that first. if you're still interested in it, we can extend the test case in the the parameterization way I mentioned