RinhaDeBackend::JsonParser

Hand-rolled JSON parser tailored to the FraudScoreRequest schema. Reads the request body as a Bytes slice and writes primitives directly into a ParsedRequest, with zero String allocations on the happy path.

Field key dispatch: at every nested object level the first letter of the key uniquely identifies the field, except at the top level where 't' collides between "transaction" (len 11) and "terminal" (len 8). We disambiguate by length there, no second-char check needed.

The parser is permissive about whitespace, strict about overall shape: malformed input raises ParseError, which the caller turns into the legit fallback (HTTP 200 with fraud_score=0.0).