Add test for CardBrowserFragmentViewModel.openDeckSelectionDialog

[GuTS805]

Covers the default usage: calling openDeckSelectionDialog() emits from flowOfSearchForDecks.
Verified via mutation testing that removing the emit breaks the test.

Fixes part of #13283

Purpose / Description

CardBrowserFragmentViewModel.openDeckSelectionDialog had no test coverage
(@NeedsTest("default usage")). This is another item from the #13283 tracker — only this single
method is covered here, per the maintainer's stated preference to keep PRs to one test at a time.

Approach

Added CardBrowserFragmentViewModelTest using Turbine to verify that calling
openDeckSelectionDialog() causes flowOfSearchForDecks to emit a value.

How Has This Been Tested?

Ran the new test in isolation and confirmed it fails when the emit is removed from
openDeckSelectionDialog() (mutation test: temporarily removed flowOfSearchForDecks.emit(Unit),
confirmed the test failed with a Turbine timeout, then reverted). Also ran the full
com.ichi2.anki.browser.* test package with no regressions.

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[github-actions]

Snapshot diff report vs main. Open screenshot-diff for diffs.

• DeckPickerScreenshotTest: 2 changes

All 2 changed screenshots

DeckPickerScreenshotTest

• hierarchy_lines_collapsed_compare.png
• hierarchy_lines_many_siblings_compare.png