docs(taskprocessing): document lockTask semantic change

Per review feedback: note in the lockTask docblock that the guard changed from
`status != RUNNING` to `status = SCHEDULED`, and that callers must now treat a
0 return as "the task is no longer claimable" rather than assuming success.

Signed-off-by: Yoan Bozhilov <bygadd@gmail.com>
Assisted-by: Claude Code:claude-opus-4-8

Author: bygadd

lib/private/TaskProcessing/Db/TaskMapper.php

@@ -381,6 +381,11 @@ public function update(Entity $entity): Entity {
 	 * worker receives the task already RUNNING, so the later SCHEDULED -> RUNNING edge in
 	 * Manager::setTaskStatus (which used to set started_at) no longer fires.
 	 *
+	 * Semantic change: this previously guarded on `status != RUNNING`, which allowed an
+	 * already SUCCESSFUL/FAILED task to be re-locked back to RUNNING. Callers must now
+	 * treat a 0 return as "the task is no longer claimable" (it is no longer SCHEDULED)
+	 * and move on, rather than assuming the lock succeeded.
+	 *
 	 * @return int Number of rows updated: 1 if the task was claimed, 0 if it was no longer scheduled.
 	 */
 	public function lockTask(Entity $entity): int {