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>

steward/core/notifications.py

@@ -57,7 +57,8 @@ def email_body(alert: dict) -> str:
 async def _send_email(cfg: dict, alert: dict) -> dict:
     try:
         msg = EmailMessage()
-        msg["Subject"] = f"[Steward] {alert['state']} — {alert['rule_name']}"
+        # A header may not hold a line break; a rule name can (MCP takes any string).
+        msg["Subject"] = " ".join(f"[Steward] {alert['state']} — {alert['rule_name']}".split())
         msg["From"] = cfg.get("from") or cfg.get("username") or "steward@localhost"
         msg["To"] = ", ".join(cfg["recipients"])
         msg.set_content(email_body(alert))