Fix concurrency issue with Blaze local notification scheduler

[itsmeichigo]

Closes: #14011

Description

When enabling Core Data concurrency debugging, I got a crash in Blaze local notification scheduler's scheduleNoCampaignReminder method.

The reason is this method dispatches an async request to check for Blaze eligibility, and then start an observation of the storage for Blaze campaigns. This causes an issue when the observation is done on a background thread.

The fix is to ensure the method is called on the main thread. I moved all the @MainActor annotations to the protocol to also support Swift targeted concurrency check.

Steps to reproduce

• Ensure that Core Data concurrency debug is enabled in the active scheme in Xcode.
• Build and run the app.
• Log in to a store eligible for Blaze.
• Confirm that the app doesn't crash on the dashboard screen.
• Do a smoke test following steps in [Blaze] Abandoned campaign creation reminder #13950 and [Blaze] No campaign reminder #13816 to confirm that local notifications are still scheduled correctly.

Testing information

Tested on simulator iPad 10th gen iOS 17.4 and confirmed that the app no longer crashes in debug mode on dashboard screen.

Screenshots

N/A

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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	App Name	WooCommerce iOS
	Build Number	pr14012-7ec9cdd
	Version	20.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	7ec9cdd
	App Center Build	WooCommerce - Prototype Builds #10943

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[pmusolino]

Just a small nit: I think you left the ConcurrencyDebug check enabled here 🙌

[itsmeichigo]

@pmusolino I intentionally enabled that as default for now. This would only affect debug build runs. Could you share why we should avoid this?

[selanthiraiyan]

Nice catch, Huong! Thanks for fixing this.

I tested that Blaze local notifications are scheduled as before. ✅