Fix MessageManager.addMessage to only add unique messages (backport #1042)

[mergify]

Changes

This PR fixes MessageManager.addMessage() to ignore already active messages displayed to the user (for all message types, like Toast or Sticky). This API is used by various tools indirectly via IModelApp.notifications.outputMessage when AppNotificationManager is set up. This change should prevent the same message from being displayed multiple times unnecessarily.

Ideally, tools and other consumers should throttle the messages manually i.e. in mouse wheel scroll handlers.
In the future, an option argument can be added if there is a use case where multiple equal messages need to be added.

Testing

Additional unit tests were added and tested via the test app.

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This is an automatic backport of pull request #1042 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	new file:   common/changes/@itwin/appui-react/add-message-once_2024-09-23-12-30.json
	modified:   ui/appui-react/src/appui-react/messages/MessageManager.tsx
	modified:   ui/appui-react/src/test/messages/MessageManager.test.tsx

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/changehistory/NextVersion.md

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