[NET10] Messaging: Migrate from MessagingCenter to modern alternatives (Toolkit WeakReferenceMessenger)

[davidortinau]

Update Messaging guidance for .NET 10: migrate from MessagingCenter to modern alternatives

What changed

• docs/fundamentals/messagingcenter.md:
  • Added a >= net-maui-10.0 migration-first section recommending CommunityToolkit.Mvvm WeakReferenceMessenger, with minimal send/receive examples (typed message via ValueChangedMessage, registration/unregistration, and an ObservableRecipient tip).
  • Added a short “When to use events instead” section showing .NET events with Microsoft.Maui.WeakEventManager for simple local scenarios.
  • Preserved legacy MessagingCenter usage under <= net-maui-9.0.
  • Switched from page-level monikerRange to inline moniker blocks and updated ms.date/description.

Why

• Microsoft.Maui.Controls.MessagingCenter is obsolete in .NET 10 (source has [Obsolete] attribute recommending CommunityToolkit.Mvvm.Messaging.WeakReferenceMessenger). This aligns the docs with platform guidance and promotes modern, testable patterns.

References

• Toolkit Messenger: https://learn.microsoft.com/windows/communitytoolkit/mvvm/messenger
• NuGet: https://www.nuget.org/packages/CommunityToolkit.Mvvm
• Weak events: Microsoft.Maui.WeakEventManager

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/messagingcenter.md	docs/fundamentals/messagingcenter

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docs/fundamentals/messagingcenter.md	✅Succeeded	View (>=net-maui-8.0)

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Learn Build status updates of commit 6044991:

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[learn-build-service-prod]

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Learn Build status updates of commit 564817e:

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[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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Learn Build status updates of commit f1c295b:

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docs/fundamentals/messagingcenter.md	✅Succeeded	View (>=net-maui-8.0)

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