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
Introduces new database function/migration and a new public client API that depends on DB state strings; mis-migrations or unexpected state values could break polling at runtime.

Overview
Adds a new polling surface via Durable::get_task_result(task_id) that returns None for unknown tasks and otherwise exposes task state plus either the completed output payload or structured error info for failed tasks.

Implements this by adding a durable.get_task_result(queue_name, task_id) Postgres function (migration + schema.sql) that reads from the queue’s task table and joins the latest attempt run for failure_reason, plus new public types (TaskStatus, TaskPollResult, TaskErrorInfo) and a new DurableError::InvalidState for unexpected DB state strings. Includes end-to-end tests covering pending/completed/failed/cancelled/nonexistent cases and user-error round-tripping.

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[anndvision]

/merge-queue

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