fix(serde): integer map keys must be exactly a JSON integer

[yuxi-liu-wired]

Related fixes in one area, 2 commits (each with its own test).

fix: reject whitespace before a numeric map key

Map keys deserialized as i8..i64, u8..u64 or f64 went through
deserialize_number, which skips leading whitespace, so whitespace
inside the key's quotes was accepted and dropped:

from_str::<BTreeMap<u32, u8>>(r#"{" 1":2}"#)     Ok({1: 2})
from_str::<BTreeMap<i64, u8>>("{\"\t1\":2}")     Ok({1: 2})

although "\t1" is not even a valid JSON string (raw control
character), and " 1" is not the text of a number. serde_json rejects
both ("expected key to be a number in quotes"), and sonic-rs's own
i128/u128 key path already did.

The single-argument arm of deserialize_numeric_key! now makes the same
check as the two-argument arm: the first byte inside the quotes must be
a digit or -.

Test: test_numeric_key_rejects_leading_whitespace (fails before this
change).

fix: numeric map keys from a Value must be the whole key

MapKeyDeserializer (used by from_value and Value as a
Deserializer) parsed an integer key as a prefix of the key text and
ignored the rest:

let v: Value = from_str(r#"{"12abc":1,"1,2":2,"08":3}"#)?;
from_value::<BTreeMap<i64, u8>>(&v)    Ok({12: 1, 1: 2, 0: 3})

Deserializing the same JSON text into the same type is an error, and so
is serde_json's from_value. After the number, the key must be at its
end; otherwise ExpectedNumericKey.

Test: test_numeric_map_key_from_value_must_be_whole_number (fails
before this change).

Testing

Each commit adds a regression test that fails before the change. The full test suite passes and
cargo fmt --check is clean on the changed files.

Found with a differential fuzzer comparing serde_json, simd-json, sonic-rs and jiter.

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