CSI: include volume namespace in staging path (#20532)

CSI volumes are namespaced. But the client does not include the namespace in the
staging mount path. This causes CSI volumes with the same volume ID but
different namespace to collide if they happen to be placed on the same host. The
per-allocation paths don't need to be namespaced, because an allocation can only
mount volumes from its job's own namespace.

Rework the CSI hook tests to have more fine-grained control over the mock
on-disk state. Add tests covering upgrades from staging paths missing
namespaces.

Fixes: #18741

.changelog/20532.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+csi: Fixed a bug where volumes in different namespaces but the same ID would fail to stage on the same client
+```

client/allocrunner/csi_hook.go

@@ -323,6 +323,7 @@ func (c *csiHook) claimVolumes(results map[string]*volumePublishResult) error {
 
 		// populate data we'll write later to disk
 		result.stub.VolumeID = resp.Volume.ID
+		result.stub.VolumeNamespace = resp.Volume.Namespace
 		result.stub.VolumeExternalID = resp.Volume.RemoteID()
 		result.stub.PluginID = resp.Volume.PluginID
 		result.publishContext = resp.PublishContext
@@ -532,7 +533,8 @@ func (c *csiHook) unmountImpl(result *volumePublishResult) error {
 	}
 
 	return manager.UnmountVolume(c.shutdownCtx,
-		result.stub.VolumeID, result.stub.VolumeExternalID, c.alloc.ID, usageOpts)
+		result.stub.VolumeNamespace, result.stub.VolumeID,
+		result.stub.VolumeExternalID, c.alloc.ID, usageOpts)
 }
 
 // Shutdown will get called when the client is gracefully