fix: add diagnostic warning when LoadingView Source is never set

[dr1rrb]

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

When LoadingView.Source is never set (stays null), the control remains in the Loading visual state indefinitely because Source?.IsExecuting ?? true evaluates to true. There is no diagnostic output to help developers identify why the view is stuck, making
black screen issues very difficult to troubleshoot.

What is the new behavior?

LoadingView.OnApplyTemplate() now schedules a deferred check: if Source is still null after 5 seconds, a Debug.WriteLine warning is emitted indicating the view will remain in Loading state indefinitely. The warning is a no-op in Release builds and does not
change any control flow or visual behavior.

New runtime tests validate LoadingView state transitions for null Source, Source becoming not-executing, and Source set after template application.

PR Checklist

• Tested code with current supported SDKs
• Tested the changes where applicable:
  • UWP
  • WinUI
  • iOS
  • Android
  • WASM
  • MacOS
• Updated the documentation as needed:
  • General Doc Update
  • Controls Doc Update
  • Extensions Doc Update
  • controls-styles.md
  • lightweight-styling.md (LightWeight Styling Resource Keys)
• Runtime Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Associated with an issue (GitHub or internal)
• Commits must be following the Conventional Commits specification.

Other information

Files changed:

• LoadingView.cs — Deferred Debug.WriteLine diagnostic when Source stays null after template apply
• LoadingViewTests.cs — New runtime tests for LoadingView state transitions

[Copilot]

Pull request overview

This PR adds a diagnostic warning to the LoadingView control that helps developers identify when the Source property is never set, which would cause the control to remain in the "Loading" visual state indefinitely. The diagnostic emits a Debug.WriteLine() message after 5 seconds if the Source remains null, providing actionable guidance without changing any functional behavior.

Changes:

• Adds WarnIfSourceNeverSetAsync() method that schedules a deferred diagnostic warning 5 seconds after template application if Source is still null
• Modifies OnApplyTemplate() to trigger the diagnostic check
• Adds required using System.Threading.Tasks; import for async support

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[unodevops]

⚠️⚠️ The build 205729 has failed on Uno Toolkit - CI.

[unodevops]

⚠️⚠️ The build 205758 has failed on Uno Toolkit - CI.

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