Fix volume repeatedly toggling between crowded and uncrowded (#7793)

* Fix volume repeatedly toggling between crowded and uncrowded

Fixes #6712

The issue was that removeFromCrowded() was called in removeFromWritable(),
which is invoked whenever a volume temporarily becomes unwritable (due to
replica count fluctuations, heartbeat issues, or read-only state changes).

This caused unnecessary toggling:
1. Volume becomes temporarily unwritable → removeFromWritable() →
   removeFromCrowded() logs 'becomes uncrowded'
2. Volume becomes writable again
3. CollectDeadNodeAndFullVolumes() runs → setVolumeCrowded() logs
   'becomes crowded'

The fix:
- Remove removeFromCrowded() call from removeFromWritable()
- Only clear crowded status when volume is fully unregistered from
  the layout (when location.Length() == 0 in UnRegisterVolume)

This ensures transient state changes don't cause log spam and the
crowded status accurately reflects the volume's size relative to
the grow threshold.

* Refactor test to use subtests for better readability

Address review feedback: use t.Run subtests to make the test's intent
clearer by giving each verification step a descriptive name.

weed/topology/volume_layout.go

@@ -206,6 +206,7 @@ func (vl *VolumeLayout) UnRegisterVolume(v *storage.VolumeInfo, dn *DataNode) {
 
 		if location.Length() == 0 {
 			delete(vl.vid2location, v.Id)
+			vl.removeFromCrowded(v.Id)
 		}
 
 	}
@@ -400,7 +401,6 @@ func (vl *VolumeLayout) removeFromWritable(vid needle.VolumeId) bool {
 			break
 		}
 	}
-	vl.removeFromCrowded(vid)
 	if toDeleteIndex >= 0 {
 		glog.V(0).Infoln("Volume", vid, "becomes unwritable")
 		vl.writables = append(vl.writables[0:toDeleteIndex], vl.writables[toDeleteIndex+1:]...)