fix: propagate CancellationToken to inner loop in AotConverterGenerator

[thomhurst]

Summary

• Adds cancellationToken.ThrowIfCancellationRequested() at the start of the DescendantNodes() inner loop in ScanTestParameters, ensuring the generator responds to cancellation during long-running node traversals
• The outer syntax tree loop and the typesToScan loop already had cancellation checks, but the potentially large inner loop over all descendant nodes in each tree did not

Test plan

• dotnet build TUnit.Core.SourceGenerator/TUnit.Core.SourceGenerator.csproj succeeds with zero warnings and zero errors
• Existing source generator snapshot tests pass (no output change expected since this is a behavioral fix only)

Closes #4900

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

This is a clean, well-targeted fix. Adding cancellationToken.ThrowIfCancellationRequested() at the top of the DescendantNodes() inner loop is the correct and idiomatic Roslyn pattern for cooperative cancellation. The placement is right (before any work is done each iteration), the API is correct (ThrowIfCancellationRequested() is zero-allocation — just a volatile read and branch), and the fix is minimal with no behavioral side effects on generated output.

The dual-mode rule does not apply here since AotConverterGenerator is a compile-time-only concern with no reflection-mode equivalent.