Fix JsonSourceGenerator crash with predefined type aliases

[Copilot]

Description

JsonSourceGenerator throws InvalidOperationException("Unreachable") when a compilation unit contains a C# 12+ using alias targeting a predefined type (e.g., using A = int;) alongside any attribute-annotated class.

In CSharpSyntaxHelper.AddAliases, usingDirective.Name returns null for predefined type aliases since PredefinedTypeSyntax is not a NameSyntax. The null value was then passed to the GetUnqualifiedName extension method, which doesn't match any NameSyntax subtype in its switch expression and hits the default throw.

// Previously crashed the source generator
using A = int;

[System.Text.Json.Serialization.JsonSerializable(typeof(MyType))]
internal partial class JsonContext : JsonSerializerContext { }

Fix:

• Store usingDirective.Name in a local and use ?.GetUnqualifiedName() with pattern matching to safely skip null/non-NameSyntax values
• Add regression test covering int, double, object, string, nuint

The fix applies to CSharpSyntaxHelper.cs in Common/src/Roslyn/, which is shared by both the System.Text.Json and Microsoft.Extensions.Logging Roslyn 4.0 source generators.

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[eiriktsarpalis]

@copilot please use braces in the branch following convention elsewhere in the repo.

[Copilot]

Done in 063b3c9.