Feat: Add onConnectionClosed callback for external reconnection handling

lib/src/database/db.dart

@@ -471,14 +471,16 @@ class Db {
       bool tlsAllowInvalidCertificates = false,
       String? tlsCAFile,
       String? tlsCertificateKeyFile,
-      String? tlsCertificateKeyFilePassword}) async {
+      String? tlsCertificateKeyFilePassword,
+      Function? onConnectionClosed}) async {
     if (state == State.opening) {
       throw MongoDartError('Attempt to open db in state $state');
     }
 
     state = State.opening;
     _writeConcern = writeConcern;
-    _connectionManager = ConnectionManager(this);
+    _connectionManager =
+        ConnectionManager(this, onConnectionClosed: onConnectionClosed);
 
     for (var uri in _uriList) {
       _connectionManager!.addConnection(await _parseUri(uri,

lib/src/network/connection.dart

@@ -211,6 +211,7 @@ class Connection {
               if (!_closed) {
                 await _closeSocketOnError(socketError: e);
               }
+              _manager.onConnectionClosed?.call();
             },
             cancelOnError: true,
             // onDone is not called in any case after onData or OnError,
@@ -223,6 +224,7 @@ class Connection {
               if (!_closed) {
                 await _closeSocketOnError(socketError: noSecureRequestError);
               }
+              _manager.onConnectionClosed?.call();
             });
     connected = true;
     return true;

lib/src/network/connection_manager.dart

@@ -7,8 +7,9 @@ class ConnectionManager {
   final replyCompleters = <int, Completer<MongoResponseMessage>>{};
   final sendQueue = Queue<MongoMessage>();
   Connection? _masterConnection;
+  final Function? onConnectionClosed;
 
-  ConnectionManager(this.db);
+  ConnectionManager(this.db, {this.onConnectionClosed});
 
   Connection? get masterConnection => _masterConnection;