Keep unsigned jsonb integers above i64::MAX

[LucaCappelletti94]

write_jsonb_number no longer rounds a u64 past i64::MAX, since #5195 replaced that code, but its new classifier treats any character that is not an ASCII digit as float syntax, and a minus sign is one, so json!(-1) is now written as a FLOAT that json_valid rejects and diesel cannot read back. This recognizes integer text instead, an optional minus followed by digits.

With the unsigned value now an exact INT, read_jsonb_int turned out to reject it, having kept a payload only when it parsed as an i64, so jsonb('18446744073709551615') did not read back either. It now accepts any integer payload that survives without rounding.

This came out of a fuzzer I am writing for diesel's deserialization code, and this PR only adds the red test for now.

[weiznich]

Just to note this down here: This likely affects i128 and u128 as well. The fix done in #5195 should address this specific case and also the larger integers as well