Fix JsonElementStringConverter non-generic IConverter swapped casts (#175)

[chullybun]

Summary

Fixes #175. JsonElementStringConverter's non-generic IConverter.ConvertToDestination(object?)/ConvertToSource(object?) overrides cast to the wrong side's type, causing InvalidCastException or infinite recursion (StackOverflowException) for any caller going through the non-generic IConverter contract.

Root cause and audit

Audited all 4 implementers of IConverter<TSource,TDestination>. Only JsonElementStringConverter was actually broken; StringBase64Converter, EncodedStringToUInt32Converter, and TypeToJsonStringConverter<T> already had correct casts, just the same duplicated hand-rolled pattern.

Fix

Rather than just patching the one bad cast, this removes the whole duplicated pattern (as the issue suggests) so the bug class can't recur:

1. IConverterT.cs — added the two missing default interface implementations for the base IConverter.ConvertToDestination(object?) / ConvertToSource(object?) members on IConverter<TSource,TDestination>. These were previously not covered by any DIM anywhere in the hierarchy (the existing DIMs only satisfy the new-hidden slots on ISourceConverter<TSource>/IDestinationConverter<TDestination> — analogous to how IEnumerable<T> hides but doesn't implement IEnumerable.GetEnumerator()). Without this addition, deleting the structs' hand-rolled overrides would not compile.
2. Deleted the redundant hand-rolled non-generic overrides from all four converters (JsonElementStringConverter, StringBase64Converter, EncodedStringToUInt32Converter, TypeToJsonStringConverter<T>), now relying solely on the interface's default implementations.
3. Added regression tests exercising the non-generic IConverter path for all four converters (new JsonElementStringConverterTests, new EncodedStringToUInt32ConverterTests, extended StringToBase64ConverterTests), mirroring the coverage already present in TypeToJsonStringConverterTests.

Testing

• dotnet build CoreEx.sln — succeeds, 0 errors.
• dotnet test tests\CoreEx.Test.Unit — 735/735 passing on net8.0, net9.0, and net10.0.

No behavioral change for any existing strongly-typed call site; verified no call site in src/ depends on the non-generic overload being directly callable on a concretely-typed struct variable.

[Copilot]

Pull request overview

This PR fixes a defect in the non-generic IConverter object-based conversion path by moving the object-cast logic into IConverter<TSource, TDestination> default interface implementations and removing the duplicated hand-rolled overrides from the converter structs. This prevents swapped-cast regressions (like the one in JsonElementStringConverter) and adds regression coverage for the non-generic IConverter route.

Changes:

• Added missing default interface implementations on IConverter<TSource, TDestination> for IConverter.ConvertToDestination(object?) and IConverter.ConvertToSource(object?).
• Removed redundant non-generic object? overload implementations from affected converter structs, relying on the interface defaults instead.
• Added/extended unit tests to explicitly exercise conversions via the non-generic IConverter interface for the affected converters.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/CoreEx.Test.Unit/Mapping/Converters/StringToBase64ConverterTests.cs	Adds regression tests that call conversion through the non-generic IConverter interface.
tests/CoreEx.Test.Unit/Mapping/Converters/JsonElementStringConverterTests.cs	New test suite including explicit coverage for non-generic IConverter conversions (regression for #175).
tests/CoreEx.Test.Unit/Mapping/Converters/EncodedStringToUInt32ConverterTests.cs	New test suite including non-generic IConverter path coverage.
src/CoreEx/Mapping/Converters/TypeToJsonStringConverter.cs	Removes redundant non-generic object? conversion overrides in favor of interface DIMs.
src/CoreEx/Mapping/Converters/StringBase64Converter.cs	Removes redundant non-generic object? conversion overrides in favor of interface DIMs.
src/CoreEx/Mapping/Converters/JsonElementStringConverter.cs	Removes previously-buggy non-generic object? conversion overrides; relies on corrected interface DIMs.
src/CoreEx/Mapping/Converters/IConverterT.cs	Adds the missing DIM implementations for the base IConverter object-based conversion members.
src/CoreEx/Mapping/Converters/EncodedStringToUInt32Converter.cs	Removes redundant non-generic object? conversion overrides in favor of interface DIMs.