Resolve a single model name to its canonical id, prices, and priceSource

Reads the optional vendor prefix (percent-encode the separator: openai%2Fgpt-5.5), then normalizes the name (dash→dot, strips bracket annotations and trailing date suffixes) and matches against the catalog, narrowed to that vendor when one is named. resolvedKey is the canonical prefixed id. priceSource is one of: oracle-basket (current index member), oracle-catalog (priced but not a current index member), off-basket (not in catalog). Unknown models return 200 + off-basket + null prices, never 404; an unknown vendor is a 422. prices.provenance marks the input and the output separately with the catalogue's own mark for each. Accepts the model key only; never reads token or usage payload.