fix(imap client): harden IMAP client message processing and modernize test tooling

ts/classes.imapclient.ts

@@ -68,6 +68,9 @@ export class ImapClient extends plugins.events.EventEmitter {
 
         try {
             const searchResult = await this.client.search(this.filter);
+            if (!searchResult) {
+                return;
+            }
 
             // Convert searchResult to a regular array
             const searchResultArray = Array.from(searchResult);
@@ -80,6 +83,9 @@ export class ImapClient extends plugins.events.EventEmitter {
 
             // Fetch messages
             for await (const message of this.client.fetch(newUids, { envelope: true, source: true })) {
+                if (!message.source) {
+                    continue;
+                }
                 const parsed = await plugins.mailparser.simpleParser(message.source);
                 this.emit('message', parsed);
                 this.seenUids.add(message.uid);
@@ -102,4 +108,4 @@ export class ImapClient extends plugins.events.EventEmitter {
             this.emit('disconnected');
         }
     }
-}
+}