Creates a content group partition on a property. Rules are an array of { type, value } objects (prefix / contains / regex / equals) that match URLs into the group. Group names are unique per site. A regex rule that cannot compile is rejected; one that is valid but unsupported by the warehouse (a lookahead, lookbehind or backreference) or skipped as ReDoS-prone is accepted and reported in ruleWarnings. Relay those to the user, since a skipped pattern matches no pages at all.