fix(notifications): a line break in a rule name no longer stops the alert email (#5064)

The rule name goes into the Subject header, and EmailMessage refuses a
header holding a line break, so the whole email failed. The web form can't
enter one but MCP takes any string. The subject now folds whitespace.
Found by the new From-fallback test in CI run 8349.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_notifications.py

@@ -106,15 +106,17 @@ def test_smtp_send_connects_by_mode_with_a_timeout(monkeypatch, port, tls, expec
     assert smtp.calls == expected + [("login", "u", "p"), ("send", "steward@example.com")]
 
 
-async def test_email_from_address_falls_back_to_username_then_localhost(monkeypatch):
+async def test_email_from_falls_back_and_a_line_break_in_the_name_still_sends(monkeypatch):
     sent = []
-    monkeypatch.setattr(n, "_smtp_send_sync", lambda cfg, msg: sent.append(msg["From"]))
+    monkeypatch.setattr(n, "_smtp_send_sync", lambda cfg, msg: sent.append((msg["From"], msg["Subject"])))
     base = {"host": "mx", "port": 587, "recipients": ["a@example.com"]}
     for cfg in ({**base, "from": "alerts@example.com", "username": "u@example.com"},
                 {**base, "from": "", "username": "u@example.com"},
                 {**base, "from": "", "username": ""}):
         assert (await n._send_email(cfg, ALERT))["sent"]
-    assert sent == ["alerts@example.com", "u@example.com", "steward@localhost"]
+    assert [f for f, _ in sent] == ["alerts@example.com", "u@example.com", "steward@localhost"]
+    # The rule name has a line break; the subject folds it to a space rather than failing.
+    assert sent[0][1] == '[Steward] FIRING — Disk "root" on web01 \\ backslash'
 
 
 def test_email_body_names_no_data_and_the_note():