test: add tests for email loop prevention

Add tests/test_email_loop_prevention.py (47 tests) covering:
- detect_automated_message(): sender patterns (mailer-daemon, postmaster,
  noreply, …), RFC headers (Auto-Submitted, X-Auto-Response-Suppress,
  multipart/report, X-Loop, Precedence), subject heuristics (German and
  English bounce/OOO phrases), and negative cases for normal senders
- EmailAgent.handle_automated_message(): state creation, loop_escalated
  flag, one-shot admin alert, silent drop on repeat, notifier failure safety
- EmailAgent.process_message() message-count cap: at-limit still processes,
  over-limit returns "", escalation called once, duplicate prevention,
  notifier failure safety, constant value assertion
- AdminNotifier.notify_loop_escalation(): SMTP dispatch, [WARNUNG] subject,
  body content, no-CC guard, no-SMTP dev-mode guard

Add TestConversationStateLoopEscalated to test_models.py (5 tests):
default value, to_dict inclusion, True round-trip, from_dict backward
compatibility with persisted conversations missing the key.

All 168 tests pass.

https://claude.ai/code/session_01KwvR5hDPjSuJg4kvw5b5e5

tests/test_models.py

@@ -150,3 +150,45 @@ class TestConversationStateSerialization:
         d = state_with_messages.to_dict()
         assert d["messages"][0]["role"] == "user"
         assert "Hallo" in d["messages"][0]["content"]
+
+
+# ---------------------------------------------------------------------------
+# ConversationState — loop_escalated field
+# ---------------------------------------------------------------------------
+
+
+class TestConversationStateLoopEscalated:
+    def test_default_loop_escalated_is_false(self, fresh_state):
+        assert fresh_state.loop_escalated is False
+
+    def test_to_dict_includes_loop_escalated(self, fresh_state):
+        d = fresh_state.to_dict()
+        assert "loop_escalated" in d
+        assert d["loop_escalated"] is False
+
+    def test_to_dict_reflects_true_when_set(self, fresh_state):
+        fresh_state.loop_escalated = True
+        d = fresh_state.to_dict()
+        assert d["loop_escalated"] is True
+
+    def test_from_dict_restores_loop_escalated_true(self, fresh_state):
+        fresh_state.loop_escalated = True
+        restored = ConversationState.from_dict(fresh_state.to_dict())
+        assert restored.loop_escalated is True
+
+    def test_from_dict_defaults_to_false_when_key_missing(self):
+        """Older persisted conversations without the key deserialise safely."""
+        data = {
+            "conversation_id": "old@example.com",
+            "parent_email": "old@example.com",
+            "language": "de",
+            "flow_step": "greeting",
+            "registration": {},
+            "messages": [],
+            "completed": False,
+            "reminder_count": 0,
+            "last_inbound_message_id": "",
+            # loop_escalated intentionally absent
+        }
+        state = ConversationState.from_dict(data)
+        assert state.loop_escalated is False