feat: test SMTP connection on save and retain password on edit

Adds testSmtpConnection() that performs EHLO + auth via JavaMailSender
before persisting to Logto — saves fail fast with a clear error if
SMTP credentials are wrong. Password is now optional when editing:
if left blank the backend fetches the existing password from Logto's
connector config, so users can update host/port/fromEmail without
re-entering the password every time.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/email-connector-hooks.ts

@@ -15,7 +15,7 @@ export interface SmtpConfigRequest {
   host: string;
   port: number;
   username: string;
-  password: string;
+  password?: string;
   fromEmail: string;
   registrationEnabled?: boolean;
 }

ui/src/pages/vendor/EmailConfigPage.tsx

@@ -50,8 +50,12 @@ export function EmailConfigPage() {
   }
 
   async function handleSave() {
-    if (!host || !username || !password || !fromEmail) {
-      toast({ title: 'All fields are required', variant: 'error' });
+    if (!host || !username || !fromEmail) {
+      toast({ title: 'Host, username, and from email are required', variant: 'error' });
+      return;
+    }
+    if (!isConfigured && !password) {
+      toast({ title: 'Password is required for new configuration', variant: 'error' });
       return;
     }
     try {
@@ -59,7 +63,7 @@ export function EmailConfigPage() {
         host,
         port: parseInt(port, 10) || 587,
         username,
-        password,
+        password: password || undefined,
         fromEmail,
       });
       toast({ title: 'Email connector saved', variant: 'success' });
@@ -217,10 +221,10 @@ export function EmailConfigPage() {
                   onChange={(e) => setUsername(e.target.value)}
                 />
               </FormField>
-              <FormField label="Password *">
+              <FormField label={isConfigured ? 'Password' : 'Password *'}>
                 <Input
                   type="password"
-                  placeholder={isConfigured ? 'Enter new password' : 'Password'}
+                  placeholder={isConfigured ? 'Leave blank to keep current' : 'Password'}
                   value={password}
                   onChange={(e) => setPassword(e.target.value)}
                 />