Fix the settings merge for user overrides

Your settings page stores settings as flat key=value entries, with dots in keys for nesting, like profile.name=ada. A user's override is applied on top of the defaults. An override value of null means the user cleared that setting. The first version, a recursive deep merge, treated null as an object and kept the old value.

An assistant wrote apply_settings_override(base_pairs, override_pairs) to replace it. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• base_pairs: the default settings, as key=value entries.
• override_pairs: the user's overrides, as key=value entries.
• Return the merged settings as key=value strings.

Rules the tests check:

• Split each entry at its first = into a key and a value. Skip an entry with no =.
• Read base_pairs in order, then override_pairs in order. A later entry for a key replaces the earlier value, within a list and across the two lists.
• A value is replaced as a whole. tags=a over tags=x|y gives tags=a.
• An override whose value is exactly null removes the key, if it is there. A later override can set it again. In base_pairs, null is an ordinary value, and an empty value is always an ordinary value.
• A key that is only in the overrides is added.
• Return the remaining settings sorted as plain strings.