Fix typed MethodDataSource provider instances

[thomhurst]

Summary

• Distinguish test-instance data sources from external provider instance data sources in source generation.
• Keep reflection fallback from reusing the wrong test class instance when an explicit provider type is used.
• Add issue [Bug]: MethodDataSource source generator uses the wrong class #6365 runtime repro and source-generator snapshot coverage.

Root Cause

MethodDataSource<TProvider> against an instance member was emitted as an InstanceMethodDataSourceAttribute, causing the engine to create the parent test class instance and the generated factory to cast it to TProvider.

Validation

• dotnet test TUnit.Core.SourceGenerator.Tests\TUnit.Core.SourceGenerator.Tests.csproj --treenode-filter "/*/*/Tests6365/*" --no-restore
• dotnet test TUnit.Core.SourceGenerator.Tests\TUnit.Core.SourceGenerator.Tests.csproj --treenode-filter "/*/*/Tests5118/*|/*/*/InheritedTestsFromDifferentProjectTests/*" --no-restore
• dotnet test TUnit.TestProject\TUnit.TestProject.csproj --treenode-filter "/*/*/Tests/*Typed_MethodDataSource_Uses_Provider_Instance*" --no-restore
• dotnet test TUnit.TestProject\TUnit.TestProject.csproj --treenode-filter "/*/*/Tests/*Typed_MethodDataSource_Uses_Provider_Instance*" --reflection --no-restore

Closes #6365

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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