Fix inconsistent server_name usage in Semantic Kernel service

Co-authored-by: pontemonti <7850950+pontemonti@users.noreply.github.com>

Author: Copilot

libraries/microsoft-agents-a365-tooling-extensions-semantickernel/microsoft_agents_a365/tooling/extensions/semantickernel/services/mcp_tool_registration_service.py

@@ -141,7 +141,7 @@ async def add_tool_servers_to_agent(
                 await plugin.connect()
 
                 # Add plugin to kernel
-                kernel.add_plugin(plugin, server.mcp_server_name)
+                kernel.add_plugin(plugin, server_name)
 
                 # Store reference to keep plugin alive throughout application lifecycle
                 # By storing plugin references in _connected_plugins, we prevent Python's garbage collector from cleaning up the plugin objects