camel-mail: fix incorrect guard in MailConverters multipart loop

The inner while loop in toString(Multipart) was checking
multipart.getCount() (the outer method parameter) instead of the
inner MimeMultipart obtained from content. This made the guard
dead code since the outer count is always >= 1 when reached.

Fix the guard to check the inner MimeMultipart's count and apply
instanceof pattern matching to eliminate the separate cast.

Claude Code on behalf of Otavio R. Piske

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: orpiske

components/camel-mail/src/main/java/org/apache/camel/component/mail/MailConverters.java

@@ -61,8 +61,7 @@ private MailConverters() {
     @Converter
     public static String toString(Message message) throws MessagingException, IOException {
         Object content = message.getContent();
-        while (content instanceof MimeMultipart) {
-            MimeMultipart multipart = (MimeMultipart) content;
+        while (content instanceof MimeMultipart multipart) {
             if (multipart.getCount() > 0) {
                 BodyPart part = multipart.getBodyPart(0);
                 content = part.getContent();
@@ -85,11 +84,11 @@ public static String toString(Multipart multipart) throws MessagingException, IO
             for (int i = 0; i < size; i++) {
                 BodyPart part = multipart.getBodyPart(i);
                 Object content = part.getContent();
-                while (content instanceof MimeMultipart) {
-                    if (multipart.getCount() < 1) {
+                while (content instanceof MimeMultipart mimeMultipart) {
+                    if (mimeMultipart.getCount() < 1) {
                         break;
                     }
-                    part = ((MimeMultipart) content).getBodyPart(0);
+                    part = mimeMultipart.getBodyPart(0);
                     content = part.getContent();
                 }
                 // Perform a case-insensitive "startsWith" check that works for different locales