Make AI features (AI Copilot + MCP) optional via [ai-features] extra

- Move fastmcp from core requirements.txt to mcp-requirements.txt
- Add MCP_AVAILABLE feature flag in agent/__init__.py (graceful degradation)
- Guard MCP imports/registration in server.py and tasks.py
- Replace ai-copilot/mcp/ai-support extras with single ai-features extra
- Add stub MCP routes returning 501 when MCP is not installed
- Add gns3server-uninstall-ai-features CLI command
- Remove old gns3server-uninstall-ai-copilot command
- Update all error messages and docs to reference ai-features

Closes #2794

gns3server/core/tasks.py

@@ -84,9 +84,12 @@ async def startup(app: FastAPI) -> None:
         m = module.instance()
         m.port_manager = PortManager.instance()
 
-    # Mark MCP server as ready to accept connections
-    from gns3server.api.routes.mcp import set_mcp_server_ready
-    set_mcp_server_ready(True)
+    # Mark MCP server as ready to accept connections (if MCP is available)
+    from gns3server.agent import MCP_AVAILABLE
+
+    if MCP_AVAILABLE:
+        from gns3server.api.routes.mcp import set_mcp_server_ready
+        set_mcp_server_ready(True)
     log.info("GNS3 server startup completed")