PATCH /api/brand-voice/{company_id} — JSON:API field-level partial update.

Per https://jsonapi.org/format/#crud-updating-resource-attributes, only
the top-level attributes present in the request body are replaced;
missing top-level fields are preserved. A client can patch just
tone_descriptors without disturbing language_patterns, and vice
versa.

A present top-level field is replaced wholesale (no recursive merge
into the list contents) — the underlying service treats
tone_descriptors and language_patterns as full-list replacements,
so partial-list edits must be done client-side by reading, modifying,
and writing the full list.

The path id must equal the auth context's company id; mismatches
return 404.