fix postgresql: validate bit length in bitstring parser

[netliomax25-code]

1. The bit/bit varying binary parser (BufferParser for BitStringRefWrapper in postgresql/include/userver/storages/postgres/io/bitstring.hpp) reads the field's bit-length prefix into an Integer (int32) directly from the server buffer, so bit_count is peer-controlled.
2. It then computes the byte count as (bit_count + 7) / 8 before comparing against the remaining buffer length. For a value near INT_MAX this addition is signed overflow (undefined behavior, reported by the UBSan build), and a negative bit_count passes the length check and skips the read loop, so a malformed field is silently accepted as an empty bit string instead of being rejected.
3. Reject bit_count < 0 up front and perform the ceil-division in size_t, so the value is validated before use and the addition cannot overflow. Parsing of valid lengths is unchanged, and both bit/bit varying and the Flags/bitset/array containers route through this parser.

Added a non-DB unit test that feeds a negative and an INT_MAX length prefix (both now throw InvalidBitStringRepresentation) plus a valid round-trip.