plugin scheduler: run iceberg and lifecycle lanes concurrently (#8821)

* plugin scheduler: run iceberg and lifecycle lanes concurrently

The default lane serialises job types under a single admin lock
because volume-management operations share global state. Iceberg
and lifecycle lanes have no such constraint, so run each of their
job types independently in separate goroutines.

* Fix concurrent lane scheduler status

* plugin scheduler: address review feedback

- Extract collectDueJobTypes helper to deduplicate policy loading
  between locked and concurrent iteration paths.
- Use atomic.Bool instead of sync.Mutex for hadJobs in the concurrent
  path.
- Set lane loop state to "busy" before launching concurrent goroutines
  so the lane is not reported as idle while work runs.
- Convert TestLaneRequiresLock to table-driven style.
- Add TestRunLaneSchedulerIterationLockBehavior to verify the scheduler
  acquires the admin lock only for lanes that require it.
- Fix flaky TestGetLaneSchedulerStatusShowsActiveConcurrentLaneWork by
  not starting background scheduler goroutines that race with the
  direct runJobTypeIteration call.

weed/admin/plugin/scheduler_lane_test.go

@@ -27,6 +27,26 @@ func TestAllLanesHaveIdleSleep(t *testing.T) {
 	}
 }
 
+func TestLaneRequiresLock(t *testing.T) {
+	tests := []struct {
+		name string
+		lane SchedulerLane
+		want bool
+	}{
+		{"Default", LaneDefault, true},
+		{"Iceberg", LaneIceberg, false},
+		{"Lifecycle", LaneLifecycle, false},
+		{"Unknown", "unknown_lane", true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := LaneRequiresLock(tt.lane); got != tt.want {
+				t.Errorf("LaneRequiresLock(%q) = %v, want %v", tt.lane, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestKnownJobTypesInMap(t *testing.T) {
 	// Ensure the well-known job types are mapped. This catches drift
 	// if a handler's job type string changes without updating the map.