feat: send MSC4075 notification event for group calls

Author: coder-with-a-bushido

lib/src/voip/group_call_session.dart

@@ -132,12 +132,28 @@ class GroupCallSession {
       await backend.initLocalStream(this, stream: stream);
     }
 
-    await sendMemberStateEvent();
+    bool shouldSendNotification = false;
+    if (!room.hasActiveGroupCall(voip)) {
+      shouldSendNotification = true;
+    }
+
+    final memberEventId = await sendMemberStateEvent();
 
     setState(GroupCallState.entered);
 
     Logs().v('Entered group call $groupCallId');
 
+    if (shouldSendNotification) {
+      Logs().d(
+        'Sending RTC notification for group call started with membership: $memberEventId',
+      );
+      await room.sendRtcNotification(
+        type: RtcNotificationType.notification,
+        mentionRoom: true,
+        memberEventId: memberEventId,
+      );
+    }
+
     // Set up _participants for the members currently in the call.
     // Other members will be picked up by the RoomState.members event.
     await onMemberStateChanged();
@@ -167,7 +183,7 @@ class GroupCallSession {
     setState(GroupCallState.ended);
   }
 
-  Future<void> sendMemberStateEvent() async {
+  Future<String?> sendMemberStateEvent() async {
     // Get current member event ID to preserve permanent reactions
     final currentMemberships = room.getCallMembershipsForUser(
       client.userID!,
@@ -238,6 +254,7 @@ class GroupCallSession {
         }
       }),
     );
+    return newEventId;
   }
 
   Future<void> removeMemberStateEvent() {