Fix the round-trip check for a config serializer

An AI wrote a property test for your config serializer that only checked that deserializing returns something, so it passed on every input. The team wants the real round-trip check: serialize, deserialize, then compare. The current format joins key=value pairs with ; and escapes nothing.

An assistant wrote roundtrip_mismatches(entries). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• entries: the config's entries, in order. Each is usually "key=value", but an entry can lack = or be empty.
• Return the keys the round trip gets wrong, as a sorted list of strings.

Rules the tests check:

• Build the config. For each entry, the key is the text before the first = and the value is everything after it. With no =, the whole entry is the key and the value is "". Keys can be empty and can contain ;. Values can contain ; and =.
• A repeated key keeps its first position and takes its last value.
• Serialize: write each key as key=value, in config order, and join them with ;. Nothing is escaped.
• Deserialize: an empty blob gives an empty config. Otherwise split the blob on every ; and read each part the same way as an entry, again with the last value winning.
• Report a key in any of these cases:
  • it is in the config but missing after the round trip
  • it is in both, with different values
  • it appears after the round trip but was not in the config
• Return each reported key once, in ascending order by plain string comparison. Case matters, and the empty key sorts first. No mismatches returns an empty list.