.Net: fix(functions): preserve JSON converter errors - #14576

Summary

Fixes #14575.

TryToDeserializeValue currently catches JsonException and falls back to the original argument, so converter validation messages are lost and reflection returns a generic type mismatch. Some connectors also provide a plain string without JSON quotes, preventing valid string-backed custom types from reaching their converter.

• Keep syntactically valid JSON as-is, and let JsonException from deserialization or a custom converter reach the caller.
• Encode a plain string as a JSON string only when it is not valid JSON.
• Preserve the caller's trailing-comma, comment, and depth settings while checking the string input.

This is distinct from #14001, which supplies default serializer options but retains the exception fallback.

Validation

• dotnet test dotnet/src/SemanticKernel.UnitTests/SemanticKernel.UnitTests.csproj --framework net10.0 --no-restore --filter 'FullyQualifiedName~KernelFunctionFromMethodTests' --logger 'console;verbosity=minimal' — 91 passed.
• dotnet format dotnet/src/SemanticKernel.Core/SemanticKernel.Core.csproj --include dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs --verify-no-changes --no-restore — passed.
• dotnet format dotnet/src/SemanticKernel.UnitTests/SemanticKernel.UnitTests.csproj --include dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs --verify-no-changes --no-restore — passed.
• Four issue-specific regression cases failed on the original implementation and pass with this change.