A literal "null"/"none" is only a mistake where the argument is a filter, so
gate the coercion on read-only query tools; a tool that writes keeps the value,
which stops update_note(content="none") from being read as "leave unchanged".
Stop dropping nullish entries from a notes tag filter too: tags are free-form,
so a literal "none" tag stays filterable and mixed tag queries keep every
branch.
Models routinely pass the literal string "null" or "none" instead of
omitting an optional argument. Taken at face value it becomes a filter
that matches nothing, so tools like list_notes / list_reports /
list_requests silently return no results.
Coerce such values to None in the central argument-coercion layer, but
only for parameters the schema allows to be null (or that are absent from
a declared "required" list), so required strings keep the literal value.
The list/filter helpers normalize the same values too, so a direct call
can't regress.
