fix(jobs): make completed->completed transition idempotent to prevent race errors (#1068)

* fix(jobs): make completed->completed transition idempotent to prevent race errors

Both execution_loop and the worker wrapper in execute() can race to call
mark_completed(). Previously the second call hit "Cannot transition from
completed to completed" and errored the job despite successful completion.

This narrowly allows only the Completed->Completed self-transition as
idempotent (early return with debug log, no duplicate history entry).
All other self-transitions remain rejected to preserve state machine
strictness.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: fix assert! formatting in idempotent completion test

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/worker/job.rs

@@ -1591,7 +1591,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn test_mark_completed_twice_returns_error() {
+    async fn test_mark_completed_twice_is_idempotent() {
         let worker = make_worker(vec![]).await;
 
         worker
@@ -1612,11 +1612,22 @@ mod tests {
             .unwrap();
         assert_eq!(ctx.state, JobState::Completed);
 
+        // Second mark_completed should succeed (idempotent) rather than
+        // erroring, matching the fix for the execution_loop / worker wrapper
+        // race condition.
         let result = worker.mark_completed().await;
         assert!(
-            result.is_err(),
-            "Completed → Completed transition should be rejected by state machine"
+            result.is_ok(),
+            "Completed -> Completed transition should be idempotent"
         );
+
+        // State should still be Completed
+        let ctx = worker
+            .context_manager()
+            .get_context(worker.job_id)
+            .await
+            .unwrap();
+        assert_eq!(ctx.state, JobState::Completed);
     }
 
     /// Build a Worker with the given approval context.