Fix spinner delay async wait

[Widthdom]

Summary

• Replaced the spinner loop's synchronous Task.Delay(...).Wait(...) with an awaited delay inside the background task.
• Added regression coverage that checks StartSpinner-related generated methods do not call Task.Wait.
• Added bilingual changelog fragment changelog.d/unreleased/1421.fixed.md.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter ConsoleUiTests -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj -- check
• Adversarial review: No blocking/actionable issues found.

Full-suite note: dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted before the latest origin/main merge and surfaced an unrelated existing failure in SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols on both net8/net9; the run was not used as final validation for this scoped fix.

Documentation and Changelog

• Changelog fragment: changelog.d/unreleased/1421.fixed.md
• No README or guide updates were required because the CLI surface and documented command contract did not change.

Fixes #1421