* Treat literal 'null'/'none' strings as absent for optional tool args
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.
* Limit nullish coercion to query tools and keep literal tags
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.
