Decouple WPAccount from WordPressAuthenticationManager

[mokagio]

Part of #24165. See also p1743540472163989-slack-C08HQR4C2TS.

There's logic in WPAccount that generates an API client and checks the associated state. If the client token is "invalid", it presents a dedicated sign-in screen to re-authenticate.

This is a bit of a blocker for moving WPAccount to WordPressData, because of all the dependencies that come with the authentication process, not to mention the fact that authentication should be unrelated with WordPressData.

To decouple the two, we can make WPAccount post a notification instead of directly triggering the sign-in presentation.

I don't know how to test this in the real world, but I covered the new logic with unit tests where possible.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.9. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	27156
	Version	PR #24392
	Bundle ID	org.wordpress.alpha
	Commit	a6a94f4
	Installation URL	4hor8llih9n3g

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📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	27156
	Version	PR #24392
	Bundle ID	com.jetpack.alpha
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	Installation URL	1hsga40m2kj20

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[mokagio]

Kept trying to figure out what the issues was with the unit tests getting stuck after my change...

I thought it had to do with the keychain warning in the console.

Then I finally thought to look at the Simulator (🤦‍♂️ for not thinking of it straightaway)

I'll timebox 30 mins for this. If I don't figure it out, I'll just remove the test in the interest of moving forward with #24165.

[crazytonyli]

I don't think we should delete this dispatch async call. We need to either post the notification from the main thread or respond to the notification from the main thread.

[kean]

wordPressComRestApi gets accessed from the background threads.

[mokagio]

I went for handling the notification on the main thread asynchronously.

I thought of using a block to leverage the queue: .main argument, 297481e, but it adds the need to store the observer and then remove it. Using the #selector is not as compact, but requires remembering less things, 4023911