feat: ignore client-supplied POP3 credentials

POP3 clients may be required to send USER/PASS, but the proxy never uses
them. Accept any client credentials blindly and always authenticate to the
IMAP backend with the configured BACKEND_IMAP_USER / BACKEND_IMAP_PASS.

Remove the previous fallback that connected to the backend using
client-supplied credentials when backend credentials were unset; the proxy
now raises a clear configuration error in that case.

Add tests covering that client credentials are ignored and that missing
backend credentials are reported.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

proxy_server.py

@@ -253,30 +253,31 @@ class POP3Session:
         return await self.send_line("-ERR Unsupported command")
 
     async def handle_user(self, args):
-        if len(args) != 1:
-            return await self.send_line("-ERR USER requires username")
-        self.username = args[0]
+        # Accept any username. Client credentials are intentionally ignored;
+        # some legacy clients insist on supplying them, so they are accepted
+        # blindly. The backend is always reached with the proxy's own creds.
+        self.username = args[0] if args else None
         return await self.send_line("+OK")
 
     async def handle_pass(self, args):
-        if len(args) != 1:
-            return await self.send_line("-ERR PASS requires password")
-        self.password = args[0]
+        # Accept any password. See handle_user: client credentials are
+        # accepted but never used or validated.
+        self.password = args[0] if args else None
         await asyncio.to_thread(self.authenticate)
         return await self.send_line("+OK User authenticated")
 
     def authenticate(self):
-        """Authenticate to the IMAP backend using configured credentials."""
-        if Settings.BACKEND_IMAP_USER and Settings.BACKEND_IMAP_PASS:
-            username = Settings.BACKEND_IMAP_USER
-            password = Settings.BACKEND_IMAP_PASS
-        elif self.username and self.password:
-            username = self.username
-            password = self.password
-        else:
-            raise RuntimeError("No IMAP credentials available")
+        """Authenticate to the IMAP backend using the configured proxy credentials.
 
-        backend = IMAPBackend(username, password)
+        Client-supplied POP3 credentials are deliberately ignored: the proxy
+        always connects to the backend with ``BACKEND_IMAP_USER`` /
+        ``BACKEND_IMAP_PASS``. This is by design for legacy clients that require
+        credentials to be entered even though the proxy does not use them.
+        """
+        if not (Settings.BACKEND_IMAP_USER and Settings.BACKEND_IMAP_PASS):
+            raise RuntimeError("Backend IMAP credentials are not configured")
+
+        backend = IMAPBackend(Settings.BACKEND_IMAP_USER, Settings.BACKEND_IMAP_PASS)
         backend.login()
         self._imap = backend
         # Snapshot the maildrop once; it stays static for the session lifetime