fix(email): harden adapter against malformed IMAP responses

Salvage of #2794 by @CharmingGroot, ported to the relocated
plugins/platforms/email/adapter.py:

- Guard raw_email = msg_data[0][1] against IndexError/TypeError and
  non-bytes payloads. UIDs are added to _seen_uids before fetch, so an
  exception mid-batch permanently skipped every remaining message in
  the batch — now the bad message is logged and skipped instead.
- Message-ID domain generation falls back to 'localhost' when
  EMAIL_ADDRESS lacks '@' (now via a shared _message_id_domain() helper
  covering all 3 send paths; the PR fixed 2 of 3).

tests/gateway/test_email_robustness.py

@@ -0,0 +1,109 @@
+"""Email adapter robustness against malformed IMAP responses (salvage of #2794).
+
+Validates that:
+- Malformed IMAP fetch responses are skipped instead of aborting the batch
+  (UIDs are marked seen before fetch, so an abort permanently loses messages)
+- Message-ID generation handles a missing '@' in EMAIL_ADDRESS
+"""
+
+import os
+import unittest
+import uuid
+from email.mime.text import MIMEText
+from unittest.mock import MagicMock, patch
+
+
+def _make_adapter(address="hermes@test.com"):
+    from gateway.config import PlatformConfig
+
+    with patch.dict(os.environ, {
+        "EMAIL_ADDRESS": address,
+        "EMAIL_PASSWORD": "secret",
+        "EMAIL_IMAP_HOST": "imap.test.com",
+        "EMAIL_SMTP_HOST": "smtp.test.com",
+    }):
+        from plugins.platforms.email.adapter import EmailAdapter
+
+        adapter = EmailAdapter(PlatformConfig(enabled=True))
+    return adapter
+
+
+def _raw_email(sender="user@test.com", subject="Hello"):
+    msg = MIMEText("Test body", "plain", "utf-8")
+    msg["From"] = sender
+    msg["Subject"] = subject
+    msg["Message-ID"] = f"<{uuid.uuid4().hex[:8]}@test.com>"
+    return msg.as_bytes()
+
+
+class TestImapResponseGuard(unittest.TestCase):
+    """_fetch_new_messages skips messages with unexpected IMAP structure."""
+
+    def _fetch_with(self, fetch_responses):
+        adapter = _make_adapter()
+        uids = b" ".join(
+            str(i + 1).encode() for i in range(len(fetch_responses))
+        )
+        fetch_iter = iter(fetch_responses)
+
+        def uid_handler(command, *args):
+            if command == "search":
+                return ("OK", [uids])
+            if command == "fetch":
+                return next(fetch_iter)
+            return ("NO", [])
+
+        mock_imap = MagicMock()
+        mock_imap.uid.side_effect = uid_handler
+        with patch("imaplib.IMAP4_SSL", return_value=mock_imap):
+            return adapter._fetch_new_messages()
+
+    def test_normal_response_parses(self):
+        results = self._fetch_with([("OK", [(b"1 (RFC822 {123}", _raw_email())])])
+        self.assertEqual(len(results), 1)
+        self.assertEqual(results[0]["sender_addr"], "user@test.com")
+
+    def test_none_element_skipped(self):
+        results = self._fetch_with([("OK", [None])])
+        self.assertEqual(results, [])
+
+    def test_empty_list_skipped(self):
+        results = self._fetch_with([("OK", [])])
+        self.assertEqual(results, [])
+
+    def test_bare_bytes_element_skipped(self):
+        # Single bytes item instead of a (header, payload) tuple
+        results = self._fetch_with([("OK", [b"not-a-tuple"])])
+        self.assertEqual(results, [])
+
+    def test_non_bytes_payload_skipped(self):
+        results = self._fetch_with([("OK", [(b"1", None)])])
+        self.assertEqual(results, [])
+
+    def test_malformed_does_not_abort_batch(self):
+        """A malformed response mid-batch must not lose the messages after it."""
+        results = self._fetch_with([
+            ("OK", [None]),                                # UID 1 malformed
+            ("OK", [(b"2 (RFC822 {123}", _raw_email())]),  # UID 2 fine
+        ])
+        self.assertEqual(len(results), 1)
+
+
+class TestMessageIdDomain(unittest.TestCase):
+    """Message-ID generation tolerates EMAIL_ADDRESS without '@'."""
+
+    def test_normal_address(self):
+        adapter = _make_adapter("hermes@example.org")
+        self.assertEqual(adapter._message_id_domain(), "example.org")
+
+    def test_address_without_at(self):
+        adapter = _make_adapter("not-an-email")
+        self.assertEqual(adapter._message_id_domain(), "localhost")
+
+    def test_address_trailing_at(self):
+        adapter = _make_adapter("weird@")
+        self.assertEqual(adapter._message_id_domain(), "localhost")
+
+
+if __name__ == "__main__":
+    unittest.main()