Fix: Use weak reference in ReaderCardService

[dvdchr]

Fixes #22640

The ReaderCardsStreamViewController may be already deallocated once the service object completes fetching, causing it to access an invalid unowned reference to self.

This fixes the issue by using a weak reference. With this, when the view controller deallocates early, we'll return Error.noInterests instead of crashing. For more investigation notes, refer to the description in #22640.

To test

Unfortunately, there's no sure way to reproduce the issue. However, based on the breadcrumbs available in Sentry, and the notes from the description in #22640, we can deduce a possible "path" to the crash:

Case 1: Trigger the Reader onboarding screen

• You can either create a new account or unsubscribe from all tags and then restart the app.
• Launch the Jetpack app.
• Select the Reader tab.
• Go to the Discover stream (if you're not already there). The tag selection screen should appear.
• Select several tags to subscribe, then press the primary button on the bottom.
• 🔎 Verify that the Discover stream now displays a list of posts without crashing.

Case 2: Spam-switch the Reader stream

• It might be good to use a slow network connection in this case. Or simulate it with Network Link Conditioner.
• Launch the Jetpack app.
• Select the Reader tab.
• From the Discover tab, quickly switch between the streams (e.g. Discover > Subscriptions > Discover > Saved).
• 🔎 Verify that the app does not crash.

Regression Notes

1. Potential unintended areas of impact
   Should be none. Change is minimal.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the changes.

3. What automated tests I added (or what prevented me from doing so)
   N/A. ReaderCardService is retained in ReaderCardsStreamViewController, and this bug happens on a race condition where the view controller is deallocated (for example because of switching to a different stream) before the fetch completes. This makes it rather tricky to unit test. Refactoring it now is also not feasible as this is part of a legacy code.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

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[wpmobilebot]

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22644-9918e56
	Version	24.2.1
	Bundle ID	org.wordpress.alpha
	Commit	9918e56
	App Center Build	WPiOS - One-Offs #8871

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[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22644-9918e56
	Version	24.2.1
	Bundle ID	com.jetpack.alpha
	Commit	9918e56
	App Center Build	jetpack-installable-builds #7901

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[wargcm]

I didn't see any issue with either test case. LGTM!