IMPORT INTO nextgen: add Init->Prepared async prepare flow (prepare_mode) #68580

Enhancement

Problem

IMPORT INTO can spend more time in submit-path prepare work before users see an actionable task lifecycle when the dataset is large, and some heavy initialization work may be duplicated between submit and later planning.

Proposal

Introduce a framework-level Init -> Prepared flow guarded by prepare_mode for nextgen IMPORT INTO (global-sort path in v1):

1. Task starts as pending + init with prepare_mode=required.
2. Framework runs prepare logic asynchronously while the task is still pending + init.
3. On prepare success, framework atomically persists:
   • step=prepared (new marker step)
   • updated task meta
   • updated concurrency / max_node_count
   • updated extra_params
4. Scheduler continues normal business-step progression from prepared to the first import step.

User-visible behavior

1. SHOW IMPORT JOBS shows phase preparing for prepare-enabled jobs.
2. Prepare-heavy work is moved out of the synchronous submit critical path.
3. Resource-related parameters computed during prepare are persisted before first business-step subtask scheduling.
4. Compatibility default remains unchanged (prepare_mode=disabled keeps existing flow).

Scope (v1)

1. Nextgen IMPORT INTO global-sort path only.
2. Framework support for StepPrepared and pending-step transition API.
3. Import scheduler/planner integration with prepared external metadata consumption.

Alternatives considered

1. Init -> Prepared (chosen): smaller blast radius, explicit durable checkpoint, no phase mutation in extra_params.
2. Init -> Preparing -> Prepared (deferred): better in-table progress observability, but higher scheduler/storage/test complexity for current one-shot prepare scope.

Acceptance criteria

1. Prepare-enabled DXF tasks follow StepInit -> StepPrepared -> first business step.
2. A dedicated pending-step transition persists prepared data atomically and tolerates benign zero-row CAS races.
3. Import jobs enter running + preparing during prepare, then transition to existing phase flow at first business step.
4. Targeted framework/import tests cover HA/race/cancel paths and pass in Ready-profile validation.

Risks / compatibility notes

1. While prepare is running, task table step remains init (in-table observability tradeoff for v1).
2. Some file-discovery-dependent failures become asynchronous failures in preparing.
3. External prepared metadata may produce orphan attempt artifacts under retries; cleanup should rely on task-scoped prefix lifecycle handling.

Reference

Design/implementation plan source:

• docs/agents/import-into/execplan-nextgen-init-prepared.md