fix(import-export): set username fallback to SharedUsername for shared servers

When loading database servers from an import file, fall back to
SharedUsername for the underlying server.username when the server is
shared and no explicit Username is provided.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: KijongHan

web/pgadmin/utils/__init__.py

@@ -733,6 +733,11 @@ def load_database_servers(input_file, selected_servers,
                 groups_added = groups_added + 1
                 groups = ServerGroup.query.filter_by(user_id=user_id)
 
+            is_shared = obj.get("Shared", None)
+            username = obj.get("Username", None)
+            if is_shared and not username:
+                username = obj.get("SharedUsername", None)
+
             # Create the server
             new_server = Server()
             new_server.name = obj["Name"]
@@ -744,7 +749,7 @@ def load_database_servers(input_file, selected_servers,
 
             new_server.port = obj.get("Port", None)
 
-            new_server.username = obj.get("Username", None)
+            new_server.username = username
 
             new_server.role = obj.get("Role", None)
 
@@ -798,7 +803,7 @@ def load_database_servers(input_file, selected_servers,
             new_server.tunnel_keep_alive = \
                 obj.get("TunnelKeepAlive", None)
 
-            new_server.shared = obj.get("Shared", None)
+            new_server.shared = is_shared
 
             new_server.shared_username = obj.get("SharedUsername", None)