[P1] Support uncertain date-only historical observations #161

The proposed API containment for imported AAA gasoline rows (api#10330) withholds unverified timestamps and exposes a date bucket with an uncertainty note. Current-main code requires a datetime in HistoricalPrice.date and maps created_at through to it; static review indicates timestamp-free rows would fail validation. This failure has not yet been executed against the SDK.

Reproduce against the proposed response, then support a date-only, explicitly uncertain observation without fabricating an intraday timestamp. Preserve existing timestamped responses and test missing/malformed dates. Coordinate release ordering with the held backend PR. Customer usage of this specific response has not been measured.