MM-57152: Get webconn count from the whole cluster (#26813)

We were setting the user status to offline without
checking for connections on other nodes in a cluster.

Now we implement a request-response mechanism for the whole
cluster and we check that before setting a user to offline.

https://mattermost.atlassian.net/browse/MM-57153

```release-note
Fix a bug where the user status would incorrectly be set to offline
without checking for connections in other nodes in an HA cluster.
```

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/einterfaces/cluster.go

@@ -29,4 +29,7 @@ type ClusterInterface interface {
 	QueryLogs(page, perPage int) (map[string][]string, *model.AppError)
 	GetPluginStatuses() (model.PluginStatuses, *model.AppError)
 	ConfigChanged(previousConfig *model.Config, newConfig *model.Config, sendToOtherServer bool) *model.AppError
+	// WebConnCountForUser returns the number of active webconn connections
+	// for a given userID.
+	WebConnCountForUser(userID string) (int, *model.AppError)
 }

server/einterfaces/mocks/ClusterInterface.go

@@ -301,6 +301,36 @@ func (_m *ClusterInterface) StopInterNodeCommunication() {
 	_m.Called()
 }
 
+// WebConnCountForUser provides a mock function with given fields: userID
+func (_m *ClusterInterface) WebConnCountForUser(userID string) (int, *model.AppError) {
+	ret := _m.Called(userID)
+
+	if len(ret) == 0 {
+		panic("no return value specified for WebConnCountForUser")
+	}
+
+	var r0 int
+	var r1 *model.AppError
+	if rf, ok := ret.Get(0).(func(string) (int, *model.AppError)); ok {
+		return rf(userID)
+	}
+	if rf, ok := ret.Get(0).(func(string) int); ok {
+		r0 = rf(userID)
+	} else {
+		r0 = ret.Get(0).(int)
+	}
+
+	if rf, ok := ret.Get(1).(func(string) *model.AppError); ok {
+		r1 = rf(userID)
+	} else {
+		if ret.Get(1) != nil {
+			r1 = ret.Get(1).(*model.AppError)
+		}
+	}
+
+	return r0, r1
+}
+
 // NewClusterInterface creates a new instance of ClusterInterface. It also registers a testing interface on the mock and a cleanup function to assert the mocks expectations.
 // The first argument is typically a *testing.T value.
 func NewClusterInterface(t interface {