fix(mail): normalize provider selection

src/vmail_client.py

@@ -356,14 +356,20 @@ class YYDSMailClient(BaseMailClient):
         raise TimeoutError(f"No OTP received within {timeout}s")
 
 
-def get_mail_client(provider: str = "vmail") -> BaseMailClient:
+def get_mail_client(provider: str | None = None) -> BaseMailClient:
     """Get mail client by provider name."""
-    if provider == "mailtm":
+    raw_provider = settings.mail_provider if provider is None else provider
+    normalized = (raw_provider or "").strip().lower()
+
+    if normalized == "vmail":
+        return VMailClient()
+    if normalized == "mailtm":
         return MailTMClient()
-    if provider == "yyds":
+    if normalized == "yyds":
         return YYDSMailClient()
-    return VMailClient()  # default to vmail
 
-
-# Default instance
-MailClient = get_mail_client("vmail")
+    if not normalized:
+        raise ValueError("MAIL_PROVIDER is empty; expected one of: vmail, mailtm, yyds")
+    raise ValueError(
+        f"Unsupported MAIL_PROVIDER={raw_provider!r}; expected one of: vmail, mailtm, yyds"
+    )