[Automation] Fire property-changed events when AutomationId changes at runtime

[dimension-zero]

Summary

• AutomationElementIdentifiers.AutomationIdProperty was missing, so runtime changes to AutomationProperties.AutomationId never raised UIA_AutomationPropertyChangedEventId on Windows (or the equivalent on macOS). UIA clients that subscribe to property changes would silently receive stale automation IDs.
• Added AutomationIdProperty sentinel to AutomationElementIdentifiers, mirroring the existing NameProperty, ItemStatusProperty, etc.
• Wired OwnerPropertyChanged in ControlAutomationPeer to raise the event when AutomationProperties.AutomationIdProperty changes (mirrors the existing ItemStatusProperty handler at line 301).
• Added AutomationIdProperty → UiaPropertyId.AutomationId to AutomationNode.s_propertyMap (Windows) so UIA_AutomationPropertyChangedEventId is fired correctly.
• Added AutomationPeer_AutomationId to AvnAutomationProperty in avn.idl and wired it in AvnAutomationPeer.s_propertyMap and automation.mm (macOS — no VoiceOver notification is posted since accessibilityIdentifier is read on-demand).
• Added unit tests in ControlAutomationPeerTests.AutomationIdNotifications covering the new notification behaviour.

Root cause analysis

GetAutomationIdCore() in ControlAutomationPeer correctly reads AutomationProperties.AutomationId ?? Owner.Name and the Windows UIA bridge correctly maps it to UIA_AutomationIdPropertyId via GetPropertyValue. Static lookups (FindElementByAccessibilityId) always worked. The gap was the change notification path: there was no AutomationIdProperty sentinel in AutomationElementIdentifiers, so it could not be added to the platform property maps, and no OwnerPropertyChanged handler raised the event. Dynamic bindings (e.g. generated IDs in ItemTemplate) would silently produce stale IDs for UIA clients watching property changes.

Test plan

• Existing Appium button tests pass: tests/Avalonia.IntegrationTests.Appium/ButtonTests.cs — all five tests use FindElementByAccessibilityId via the x:Name fallback and must continue to resolve correctly.
• Existing AutomationTests.AutomationId() test passes — verifies explicit AutomationProperties.AutomationId takes precedence over x:Name.
• New unit tests pass: ControlAutomationPeerTests.AutomationIdNotifications (3 tests) — verify PropertyChanged fires on the peer when AutomationProperties.AutomationId is set at runtime.
• macOS: build Avalonia.Native on macOS to confirm the avn.idl / automation.mm changes compile (requires macOS CI).

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065449-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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[dimension-zero]

@dimension-zero,

Please read the following Contributor License Agreement (CLA). If you agree with the CLA, please reply with the following:

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[dimension-zero]

dimension-zero doesn't seem to be a GitHub user.
Please add the emails used in your commits to your Github account accordingly.

Done.

[dimension-zero]

All checks have passed. Can't add "bug" label as not an owner/admin.

[MrJul]

LGTM!

[dimension-zero]

LGTM!

HTH!