Tests: Coverage for JsonDateTimeFormat attribute behavior and DateTimeConverterResolver

[Copilot]

Adds missing test coverage demonstrating that [JsonDateTimeFormat] requires DateTimeConverterResolver to function, and increases coverage of DateTimeConverterResolver constructors and fallback paths.

Changes

New test file: ReflectionBasedTests.JsonDateTimeFormat.cs

• Verifies [JsonDateTimeFormat] is ignored in reflection-based serialization without DateTimeConverterResolver
• Demonstrates default formats are used instead of attribute-specified formats

Enhanced: SourceGeneratorBasedTests.JsonDateTimeFormat.cs

• Added tests showing source generators ignore [JsonDateTimeFormat] without DateTimeConverterResolver
• Covers both single-parameter and context-based serialization paths

New test file: DateTimeConverterResolverTests.cs

• Covers parameterless DateTimeConverterResolver() constructor
• Covers DateTimeConverterResolver(JsonSerializerOptions) constructor
• Tests GetTypeInfo(Type) fallback path when _source is null
• Tests GetTypeInfo(Type) when _source is JsonSerializerContext vs other resolvers

Example

// Without DateTimeConverterResolver, custom format is ignored
var options = new JsonSerializerOptions { WriteIndented = true };
var model = new ModelWithAttribute { DateTimeProperty = new DateTime(2023, 10, 1, 12, 0, 0, DateTimeKind.Utc) };
var json = JsonSerializer.Serialize(model, options);
// Result: "2023-10-01T12:00:00Z" (default format, not custom "yyyy-MM-ddTHH:mm:ss")

// With DateTimeConverterResolver, custom format is applied
var resolverOptions = new JsonSerializerOptions
{
    TypeInfoResolver = new DateTimeConverterResolver(ResolverContext.Default)
};
json = JsonSerializer.Serialize(model, resolverOptions);
// Result: "2023-10-01T12:00:00" (custom format from attribute)

Original prompt

I think we need to add
ReflectionBasedTests.JsonDateTimeFormat.cs
that verifies that [JsonDateTimeFormat("xxx)] verifies that reflection based serlize / deserlize doesn't work with it, and no DateTimeConverterResolver should be used.

I also would add additional tests to SourceGeneratorBasedTests.JsonDateTimeFormat.cs that shows that without using DateTimeConverterResolver it doesn't work

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.07%. Comparing base (709d47b) to head (76f5f83).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   92.30%   98.07%   +5.76%     
==========================================
  Files          13       13              
  Lines         156      156              
  Branches       35       35              
==========================================
+ Hits          144      153       +9     
+ Misses          7        1       -6     
+ Partials        5        2       -3     

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