Test: Cover DateTimeConverterFactoryHelper and nullable converter WriteNullValue paths

[Copilot]

Codecov identified two uncovered code paths: the NotSupportedException in DateTimeConverterFactoryHelper.CreateConverter for unsupported types, and the writer.WriteNullValue() branch in all nullable converters.

Changes

• Added InternalsVisibleTo attribute to main project csproj for test access to internal classes
• Created DateTimeConverterFactoryHelperTests.cs with 11 tests covering:
  • All 8 supported type conversions (DateTime, DateTimeOffset, DateOnly, TimeOnly + nullables)
  • NotSupportedException for unsupported types (string, int)
  • ArgumentNullException for null type parameter
• Created NullableConverterWriteTests.cs with 12 tests directly invoking converter Write methods:
  • WriteNullValue() path for all 4 nullable converters
  • Write with actual values to ensure both branches covered
  • ArgumentNullException when writer is null
• Fixed CA1869 analyzer warning in ConverterTests.cs by using unique date format strings per test instead of reusing identical JsonSerializerOptions instances

Test Results

88 tests passing (+23 from baseline of 65)

Original prompt

Codecov shows that DateTimeConverterFactoryHelper.CreateConverter doesn't cover

throw new NotSupportedException($"{typeToConvert.FullName} is not supported by the {nameof(DateTimeConverterFactoryHelper)}.");

add:



To test it, actually test it fully and check if the correct type instance is returned

Also codecov shows that DateOnlyNullableConverter.cs, DateTimeNullableConverter.cs, DateTimeOffsetNullableConverter.cs, TimeOnlyNullableConverter.cs
doesn't fully cover the Write when it's null aka the writer.WriteNullValue(); add tests for this too

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (4ee291c) to head (0269fdb).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   83.33%   92.30%   +8.97%     
==========================================
  Files          13       13              
  Lines         156      156              
  Branches       35       35              
==========================================
+ Hits          130      144      +14     
+ Misses         16        7       -9     
+ Partials       10        5       -5     

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