Reader: Fix tag stream wiping saved posts on pull-to-refresh

[dvdchr]

Cherry-picked from #23317

When performing a pull-to-refresh on the tag stream, the service wipes out all posts associated with the tag, and this could also delete saved posts (i.e., data loss).

Root cause: ReaderPostStreamService is used to fetch data for tag streams. When fetching data for the first page, the service wipes all data related to the tag, but there's no check in place for posts that are in use or saved.

fbf98ca solves this by skipping posts that are in use (inUse == true) or saved (isSavedForLater == true) when deleting posts associated to the tag. The fetch operation merges the new remote object with existing objects based on the globalID property (see ReaderPost.createOrReplace). Since inUse and isSavedForLater is purely a client-side property, it won't be overridden by the values from remote.

To test

Run the unit tests and ensure they pass 🟢 . In addition, here are the manual testing steps for each scenario:

1. Pre-requisite: Ensure that you at least follow one tag
2. Launch the Jetpack app
3. Navigate to Reader
4. Go to the 'Your Tags' stream
5. On one of the tags, scroll to the end and tap 'More'
6. In the Tags stream page, tap the ellipsis button on the first post and tap 'Save'
7. Perform a pull-to-refresh
8. Find the post that you've saved before
9. 🔎 Verify that when tapping the ellipsis menu, the menu item says 'Remove Saved Post' (or the equivalent in your locale)
10. Go to the 'Saved' stream from the stream switcher
11. 🔎 Verify that the post is visible in this stream

Regression Notes

1. Potential unintended areas of impact
   Should be none.

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)
   Added unit tests to cover the specific scenarios covered by this PR.

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
⚠️	This PR is assigned to the milestone 25.1 ❄️. 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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[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	pr23331-9e57051
	Version	25.0
	Bundle ID	com.jetpack.alpha
	Commit	9e57051
	App Center Build	jetpack-installable-builds #9168

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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	pr23331-9e57051
	Version	25.0
	Bundle ID	org.wordpress.alpha
	Commit	9e57051
	App Center Build	WPiOS - One-Offs #10119

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[wargcm]

LGTM!

[wargcm]

Nice 💯, I like this over using an expectation.