feat: add ViewModel progress infrastructure [DEV]

[criticalAY]

Purpose / Description

Introduce a common pattern for progress notifications in ViewModels, decoupling progress dialog management from Activity/Fragment context.

Fixes

• Fixes part of architecture: support withProgress in ViewModels #19460

Approach

• The ViewModel exposes a state flow. The Activity/Fragment calls observeProgress(viewModel) once, and the LoadingDialogFragment is shown/dismissed automatically based on state changes.
• Cancellation follows the same pattern as the old API (pass an onCancel callback to withProgress). The observer wires it to the dialog's cancel listener. (I actually followed a different pattern here then I saw David's comment on original issue)

How Has This Been Tested?

Unit test

Learning (optional, can help others)

NA

Checklist

Please, go through these checks before submitting the PR.

• 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

[david-allison]

This looks exceptional. One question on the race condition

[david-allison]

I feel we should store each onCancel.

This would require arbitrary access to the list of currently active methods, as they are removed based on the completion time.

[david-allison]

Consider using launchCollectionInLifecycleScope

[david-allison]

Sorry... I was incorrect here. You need a lock when updating the progress state, but this isn't an update method.

[david-allison]

dialogVisible doesn't restore state from the FragmentManager

[david-allison]

Concurrency bugs here: There's dependencies between currentOnCancel, activeCount and progress, but locks aren't held to serialise the mutations.

[david-allison]

Thinking more:

What's does progress/cancellation mean if there are two concurrent progress operations.

Let's define this explicitly, both the UI and UX.