Add get_task_result() polling API

[anndvision]

Summary

• Adds get_task_result() to Durable<State> for polling task status and retrieving output/error info after spawning
• New types: TaskStatus, TaskPollResult, TaskErrorInfo
• New durable.get_task_result stored procedure (migration + schema.sql)
• New InvalidState variant on DurableError

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Note

Medium Risk
Adds a new Postgres stored procedure and a new public client API surface for querying task status/output, so compatibility depends on running the new migration and on state string parsing staying aligned with DB constraints.

Overview
Adds a polling API (Durable::get_task_result) to fetch a task’s current TaskStatus and, when terminal, its output (completed_payload) or failure details (failure_reason).

Implements this via a new durable.get_task_result(queue_name, task_id) PL/pgSQL function (migration + schema.sql) and new public types (TaskStatus, TaskPollResult, TaskErrorInfo), plus a new DurableError::InvalidState for unexpected DB state strings.

Adds integration tests covering polling for pending/completed/failed/cancelled and nonexistent tasks, including user-error vs step-error failure payloads.

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[anndvision]

/merge-queue

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