Lookup Bills

Response for the batch lookup.

not_found is the point of the shape. A batch endpoint that just returns the rows it managed to resolve makes a typo, a renumbered bill and a jurisdiction we do not cover all look identical to a caller diffing counts -- and this project has already shipped two bugs where a short result was read as a complete one. Every requested key is accounted for in exactly one of the three fields.

ambiguous is kept separate from not_found because they call for opposite actions. Not-found means the bill is not here; ambiguous means it is here more than once (the same number in several sessions) and the request needs a session to pick one. It maps the key to the session identifiers that matched, so the caller can re-ask without a second query. Folding these together would tell a consumer to drop a bill we hold.