Enlighten GenerateTrustInfo task for multithreaded mode

[jankratochvilcz]

Enlighten GenerateTrustInfo task for multithreaded mode

Parent: #11834

Context

GenerateTrustInfo has two implementations via conditional compilation:

• NETFRAMEWORK: reads BaseManifest.ItemSpec and writes TrustInfoFile.ItemSpec — both file I/O with potentially relative paths
• .NET (non-Framework): inherits from TaskRequiresFramework and just returns false with an error

The cwd-dependence is in the NETFRAMEWORK path: FileSystems.Default.FileExists(BaseManifest.ItemSpec), trustInfo.ReadManifest(BaseManifest.ItemSpec), and trustInfo.Write(TrustInfoFile.ItemSpec).

Approach

1. Mark both GenerateTrustInfo classes with [MSBuildMultiThreadableTask] (Inherited=false requires it on each concrete class)
2. Implement IMultiThreadableTask on the NETFRAMEWORK version with path absolutization for BaseManifest.ItemSpec and TrustInfoFile.ItemSpec
3. The .NET version only needs the attribute (no file ops)
4. Use OriginalValue in error messages (Sin 2)

Acceptance criteria

• Both GenerateTrustInfo classes decorated [MSBuildMultiThreadableTask]
• NETFRAMEWORK version implements IMultiThreadableTask with path absolutization
• .NET version: attribute-only (no IMultiThreadableTask needed)
• Error messages use original paths (Sin 2)
• All existing tests pass
• No new compiler warnings

References

• Parent epic: Migrate Tasks in the MSbuild repo to the new Task type #11834
• SKILL: .github/skills/multithreaded-task-migration/SKILL.md