refactor: update gateway agent session key handling and improve related logic

frontend/src/components/gateways/GatewayForm.tsx

@@ -9,7 +9,7 @@ type GatewayFormProps = {
   name: string;
   gatewayUrl: string;
   gatewayToken: string;
-  mainSessionKey: string;
+  mainSessionKey: string | null;
   workspaceRoot: string;
   gatewayUrlError: string | null;
   gatewayCheckStatus: GatewayCheckStatus;
@@ -17,7 +17,6 @@ type GatewayFormProps = {
   errorMessage: string | null;
   isLoading: boolean;
   canSubmit: boolean;
-  mainSessionKeyPlaceholder: string;
   workspaceRootPlaceholder: string;
   cancelLabel: string;
   submitLabel: string;
@@ -28,7 +27,6 @@ type GatewayFormProps = {
   onNameChange: (next: string) => void;
   onGatewayUrlChange: (next: string) => void;
   onGatewayTokenChange: (next: string) => void;
-  onMainSessionKeyChange: (next: string) => void;
   onWorkspaceRootChange: (next: string) => void;
 };
 
@@ -44,7 +42,6 @@ export function GatewayForm({
   errorMessage,
   isLoading,
   canSubmit,
-  mainSessionKeyPlaceholder,
   workspaceRootPlaceholder,
   cancelLabel,
   submitLabel,
@@ -55,7 +52,6 @@ export function GatewayForm({
   onNameChange,
   onGatewayUrlChange,
   onGatewayTokenChange,
-  onMainSessionKeyChange,
   onWorkspaceRootChange,
 }: GatewayFormProps) {
   return (
@@ -137,13 +133,12 @@ export function GatewayForm({
       <div className="grid gap-6 md:grid-cols-2">
         <div className="space-y-2">
           <label className="text-sm font-medium text-slate-900">
-            Main session key <span className="text-red-500">*</span>
+            Main session key (read-only)
           </label>
           <Input
-            value={mainSessionKey}
-            onChange={(event) => onMainSessionKeyChange(event.target.value)}
-            placeholder={mainSessionKeyPlaceholder}
-            disabled={isLoading}
+            value={mainSessionKey ?? "Auto-generated by server"}
+            readOnly
+            disabled
           />
         </div>
         <div className="space-y-2">