Honor [scheduler] create_cron_data_intervals when serializing cron schedules in the Go SDK

[jason810496]

Background

The Go bundle serializer always emits CronTriggerTimetable for a cron schedule, which only matches the default deployment ([scheduler] create_cron_data_intervals = False). When a deployment sets it to True, Python's _create_timetable (task-sdk/src/airflow/sdk/definitions/dag.py) produces CronDataIntervalTimetable instead, so a Go-authored DAG diverges from the equivalent Python DAG.

The Go bundle binary cannot read airflow.cfg, so it has no way to branch on the flag today. The relevant scheduler settings must be delivered from the supervisor (Python ExecutableCoordinator) to the lang-SDK over the coordinator protocol first.

Current hardcoded branch in go-sdk/pkg/execution/serde.go (serializeTimetable):

default:
    return map[string]any{
        "__type": "airflow.timetables.trigger.CronTriggerTimetable",
        "__var": map[string]any{
            "expression":      *schedule,
            "timezone":        "UTC",
            "interval":        0.0,
            "run_immediately": false,
        },
    }

What needs to happen

1. Extend the coordinator protocol so the supervisor sends create_cron_data_intervals (and create_delta_data_intervals, once timedelta schedules are supported) to the bundle — e.g. as fields on DagFileParseRequest.
2. In serializeTimetable, emit CronDataIntervalTimetable ({expression, timezone}) when create_cron_data_intervals is True and CronTriggerTimetable when False, mirroring _create_timetable.
3. Carry the DAG timezone over the same channel; serializeTimetable currently hardcodes "UTC".

Acceptance criteria

• A cron-scheduled Go DAG serializes to the same timetable __type and __var as the equivalent Python DAG under both values of create_cron_data_intervals.
• DAG timezone is no longer hardcoded to "UTC".
• The TODO on serializeTimetable in go-sdk/pkg/execution/serde.go is removed.

Context

• Originating PR: Go-SDK: Dag/Task specs and downstream wiring in the authoring API #67155 (Go-SDK Dag/Task specs and downstream wiring in the authoring API) — serde parity review.
• Related: Cross-SDK DagSerialization compatibility CI for the Go SDK #66942 (Cross-SDK DagSerialization compatibility CI for the Go SDK) — a CI harness like that would catch this divergence automatically.
• Python reference: _create_timetable in task-sdk/src/airflow/sdk/definitions/dag.py; config default in airflow-core/src/airflow/config_templates/config.yml (create_cron_data_intervals, default False).

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Drafted-by: Claude Code (Opus 4.8); reviewed by @jason810496 before posting