Fix config source gen with value tuple

[tarekgh]

Fix #115133

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[ericstj]

LGTM
I checked to see what Json does and it has similar special casing for ValueTuple

runtime/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

Line 1474 in 27604b5

{ IsValueTuple: true } => $"() => default({typeSpec.TypeRef.FullyQualifiedName})",

[ericstj]

Interesting this is the first time we're compiling the generator output -- we didn't have any other patterns to follow here for validating this?

[tarekgh]

We do in other places, for example:

runtime/src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/LoggerMessageGeneratorParserTests.cs

Line 1029 in 27604b5

byte[] referencedImage = CompilationHelper.CreateAssemblyImage(referencedCompilation);

used to compile and run the generated code. In my case I wanted to validate it compile fine. This is used also in other generators too like regex and json.