fix: treat all-dot display as a keep-existing placeholder

The all-dot mask returned for short or un-disclosable secrets was not
recognized by is_core_config_secret_placeholder, so a client round-tripping
that display value would persist the bullet characters as a literal
credential. Recognize bullet-only values as keep-existing, matching the
frontend's isMaskedSecretValue.

Author: xxynet

main_routers/config_router/core_config.py

@@ -80,6 +80,10 @@ def is_core_config_secret_placeholder(value) -> bool:
         return True
     if len(stripped) >= 3 and set(stripped) == {'*'}:
         return True
+    # The all-dot display (short or un-disclosable secrets) must round-trip as
+    # "keep existing" too, never be persisted as a literal bullet credential.
+    if stripped and set(stripped) == {'•'}:
+        return True
 
     # Display masks keep a visible prefix/suffix and star out the middle.  Both
     # the current 3+3 shape and the legacy 6+6 shape mean "keep existing".