Source-gen silently drops a generic test method when its type arguments cannot be inferred from the data source #7163

Follow-up split out of #7108 and PR #7143.

What happens

In source-generated mode, a generic test method whose type arguments cannot be inferred at compile time is dropped silently. No test is registered and no diagnostic is reported. Reflection mode still discovers and runs the test, so the two modes disagree on the test count without any hint. Typical inputs:

• [MethodDataSource] rows typed object, object[] or a non-generic IEnumerable. The declared type says nothing about T; see Source-gen infers generic type arguments from a data source's declared type, so broad declared types fail constraints that the runtime value meets #7116 for the runtime/declared-type gap.
• A single-value row feeding Test<T1, T2>(T1 a, T2 b), where T2 cannot be inferred from the row.

PR #7143 widened the row shapes source-gen can infer from, but these cases still fall through InferTypesFromMethodDataSource (src/TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs), which returns null, and no entry is emitted.

How to reproduce

public class Repro
{
    public static IEnumerable<object> Rows() { yield return 1; }

    [Test, MethodDataSource(nameof(Rows))]
    public void Generic<T>(T value) { }
}

dotnet run shows no test. dotnet run -- --reflection shows Generic<int>.

Expected

The issue author asked for one of two things, and this tracks that remaining part of #7108. Either source-gen reports a diagnostic, for example "cannot infer type arguments for generic test X from data source Y; add [GenerateGenericTest] or type the data source", or it emits a placeholder that fails at runtime with that message. Either way, the test must not vanish silently.