fix(search): clear SelectedTags when AvailableTags is rebuilt

[jaybarden1]

Summary

When SetActiveAccount is called (account switch) or OnViewActivatedAsync rebuilds the tag list because new tags are available, all CheckBox controls in the ItemsControl are destroyed and recreated — starting with IsChecked = false. However, SelectedTags was never cleared in either path. Previously selected tags survived the rebuild invisibly and silently filtered subsequent searches even though no checkbox appeared checked.

Fix: clear SelectedTags inside the same dispatcher.Post callback that clears/rebuilds AvailableTags, in both SetActiveAccount and OnViewActivatedAsync.

Type of change

• Feature
• Bug fix
• Refactor
• CI/CD
• Documentation
• Maintenance

Related issues / links

Closes #683

How was this tested?

• Unit tests added/updated
• Manual validation
• Not applicable

Three new tests in GivenASyncedFileSearchViewModel:

• when_account_changes_then_selected_tags_are_cleared
• when_view_is_activated_again_with_more_tags_then_selected_tags_are_cleared
• when_view_is_activated_again_with_more_tags_then_search_does_not_apply_stale_selected_tags

All three were written red-first, then made green by the fix.

Impacted areas

apps/desktop/AStar.Dev.OneDrive.Sync.Client — Search/SyncedFileSearchViewModel.cs, appsettings.json (version bump 0.29.0 → 0.29.1)

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TDD Checklist (required)

• Builds locally
• Tests pass (dotnet test)
• No new analyzer warnings
• Public API changes reviewed
• Documentation updated (if applicable)
• Not applicable

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How to run tests locally

dotnet restore AStar.Dev.slnx
dotnet test --verbosity normal

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Notes for reviewers

• Root cause: CheckBox items are recreated (visual state reset) when AvailableTags is rebuilt, but SelectedTags was not cleared to match.
• The dispatcher.Post grouping ensures both collections are cleared atomically on the UI thread.