Restore Windows notification registration compatibility

[jfversluis]

Description of Change

PR #3301 made Snackbar notification initialization work on .NET 11, but its review-driven event subscription ordering broke .NET 10 with Windows App SDK 1.8. Subscribing to NotificationInvoked after Register() throws COMException 0x80070490 because Windows App SDK requires event handlers to be subscribed before registration.

This restores the required subscribe-before-Register() ordering. A local registration-success flag is set only after Register() succeeds, and a finally block removes the handler whenever registration fails, including the already-handled .NET 11 0x8007007E Insights resource DLL failure and unexpected exceptions. Successful registrations retain the handler and existing shutdown cleanup.

The ordering behavior was verified in isolation on both .NET 10 and .NET 11. The focused AppBuilderExtensionsTests suite passes, and the .NET 10 Windows library target builds successfully.

This PR remains draft while our testers verify the fix before it is marked ready for review.

Linked Issues

• Follow-up to Handle missing Windows notification runtime module #3301

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal) — follow-up regression fix for Handle missing Windows notification runtime module #3301
• Has tests (existing focused AppBuilderExtensions tests pass; platform registration ordering was verified in isolation)
• Has samples (not applicable; lifecycle registration fix only)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls — not applicable; no public API or behavior documentation change

Additional information

The regression reproduces on .NET 10 / Windows App SDK 1.8 as Element not found. Must register event handlers before calling Register(). On .NET 11, the known missing Insights resource DLL registration failure remains handled, while the pre-registration event subscription is now reliably removed.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The fix targets runtime/SDK-specific COM behavior on Windows and the PR is still marked draft pending tester verification.

Review tier: Lite
Findings: None

What changed in this PR

Restores Windows App SDK–compatible ordering for Snackbar notification registration by ensuring NotificationInvoked is subscribed before calling AppNotificationManager.Register(), preventing the .NET 10 / Windows App SDK 1.8 COMException (0x80070490) regression introduced by PR #3301.

Changes:

• Subscribe to NotificationInvoked before Register() to satisfy Windows App SDK requirements.
• Track registration success locally and reliably unsubscribe the handler in a finally block when registration fails (including the known .NET 11 0x8007007E module-missing case).

File	Description
src/​CommunityToolkit.Maui/​Options.shared.cs	Reorders Windows Snackbar notification event subscription to occur before registration and adds cleanup on registration failure.

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[jfversluis]

Our vendor team has now tested this fix with both .NET 10 and .NET 11, and Windows Snackbar notification registration works correctly with both versions.

[Copilot]

Copilot review overview

🟢 Approval recommended

The change is small, targeted, and aligns the Windows notification registration flow with Windows App SDK ordering requirements while preserving existing failure-handling behavior.

Review tier: Lite
Findings: None