feat(await): add optional timeout and poll_completion to await_completion

[HonestMajority]

Summary

• Add optional timeout: Option<Duration> parameter to await_completion — if the timeout expires before the job reaches a terminal state, returns JobError::TimedOut
• Add non-blocking poll_completion convenience method — returns the terminal state immediately if done, or None if still running
• Add JobError::TimedOut(JobId) error variant

Motivation

await_completion (added in #59) blocks indefinitely if a job never completes. Callers need a way to limit how long they wait. Additionally, some callers want a non-blocking check without subscribing to notifications.

Changes

await_completion timeout (src/lib.rs)

• Signature: await_completion(&self, id: JobId, timeout: Option<Duration>)
• None preserves existing behavior (wait indefinitely)
• Some(duration) wraps the notification wait with tokio::time::timeout
• On expiry, returns JobError::TimedOut(id)

poll_completion (src/lib.rs)

• Signature: poll_completion(&self, id: JobId) -> Result<Option<JobTerminalState>, JobError>
• Loads the job from DB and checks terminal_state()
• Returns immediately — no channel subscription needed

Error type (src/error.rs)

• Added TimedOut(JobId) variant to JobError

Tests (tests/job.rs)

• Updated existing await_completion tests to pass None for backward compat
• Added test_await_completion_timeout — verifies TimedOut error on expiry
• Added test_poll_completion — verifies None for running jobs, Some(Completed) for finished jobs

Test plan

• Existing await_completion tests pass with None
• Timeout test with short duration on a far-future job
• Poll returns None for in-progress, Some(Completed) for finished
• CI checks pass

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