fix correctness bugs across core, JsonPath, Xml and FSharp

[SimonCropp]

fix correctness bugs across core, JsonPath, Xml and FSharp

Add regression tests (src/ArgonTests/BugFixes/, NullTokenTests.fs) and fix:

core writer/reader

• JsonConvert.ToString(TimeSpan) escaped the whole stack buffer, embedding NULs
• JsonWriter.WriteValue(StringBuilder) wrote each chunk as a separate JSON string
• JsonTextWriter.WriteValue(char) ignored QuoteValue/QuoteChar
• JsonTextReader.Close() double-returned charBuffer to ArrayPool on a second call
• JsonReader.SetUndefinedToken set Null instead of Undefined

serializer

• JsonConverter.ReadJson returned null on container tokens without consuming them
• OrderByKey packed the comparer into the sort key, making ordering culture-sensitive
• JsonSerializerProxy did not forward ResolveContract or Converters to converters
• CreateProperties used unstable List.Sort, scrambling order for >16 props with Order
• removed duplicate EncodingConverter registration

utilities

• GetDefaultValue(char) returned boxed int 0 so DefaultValueHandling.Ignore missed chars
• BoxedPrimitives.Get(decimal) dropped the sign of negative zero
• ISO parser accepted out-of-range zone offsets, throwing from the DateTimeOffset ctor

linq

• CompareBigInteger inverted comparisons against negative fractions
• GetHashCode/GetDeepHashCode were inconsistent with Equals across numeric types
• TryAddInternal advanced the index when a child insert was skipped
• implemented JObject IDictionary.Values (was NotImplementedException)

jsonpath

• FieldMultipleFilter threw ErrorWhenNoMatch even when the property existed
• numeric filter literals were not terminated by && or ||
• $[-1] crashed instead of returning null

fsharp

• FSharpList/FSharpMap converters crashed on a JSON null

xml

• single-element nested arrays lost json:Array for prefixed/encoded names
• multiple sibling comments serialized to "#comment": [], dropping the text
• CreateXmlDocumentType discarded the DTD internal subset
• #cdata-section/#text with a null value crashed for XDocument