Edits an existing content group: its name, description, color, or matching rules. Only the fields you pass change; omit a field to leave it untouched. Passing rules REPLACES the whole rule list (rules have no stable ids across edits), so read the current rules with list_content_groups first if you mean to add one. Changing rules reassigns pages immediately in every report that uses the group; nothing is recomputed or stored, so the next get_content_group_performance call reflects the edit. A regex rule that cannot compile is rejected; one that is valid but unsupported by the warehouse or skipped as ReDoS-prone is accepted and reported in ruleWarnings. Relay those, since a skipped pattern matches no pages at all.