Fix saturation_factor=0 to allow task assignment to idle workers

The previous implementation returned 0 for idle workers when
saturation_factor=0, which caused _worker_full() to incorrectly
mark idle workers as full (since it checks `<= 0`). This prevented
any tasks from being assigned, causing test timeouts.

Changed _task_slots_available() to return 1 for idle workers when
saturation_factor=0, allowing idle workers to accept tasks while
still preventing queuing on busy workers.

Behavior with saturation_factor=0:
- Idle worker: slots = 1 (can accept tasks)
- Worker with 1 task: slots = 0 (marked as full)
- Worker with seceded task: slots = 1 (can accept tasks)

Fixes test_saturation_factor[0.0-expected_task_counts5] timeout.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: 2 people

distributed/scheduler.py

@@ -9289,7 +9289,7 @@ def _task_slots_available(ws: WorkerState, saturation_factor: float) -> int:
     # Special case: saturation_factor == 0 means no queuing
     # Only send tasks to workers that are completely idle
     if saturation_factor == 0:
-        return 0 - (len(ws.processing) - len(ws.long_running))
+        return 1 - (len(ws.processing) - len(ws.long_running))
 
     return max(math.ceil(saturation_factor * ws.nthreads), 1) - (
         len(ws.processing) - len(ws.long_running)