Fix unbounded growth of number media observers in Post Editor

[kean]

I was working on #21190 and noticed that the media observer callback was getting called more times than expected:

2023-08-17 10:19:40.856161-0400 Jetpack[79340:16077759] thumbnailReady
2023-08-17 10:19:40.856599-0400 Jetpack[79340:16084149] obserersCount: 21
2023-08-17 10:19:40.856992-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.857318-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.857671-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.858153-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.858397-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.858612-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.858811-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.859026-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.859256-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.859446-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.859637-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.859839-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.860153-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.860423-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.860650-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.860856-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048
2023-08-17 10:19:40.861054-0400 Jetpack[79340:16077759] mediathumbnailReady: -1680247048

It turned out, the longer you edit the post, the more media observers it adds. So, in my case, it was redrawing the image block 20 times instead of one 🫠

GutenbergMediaInserterHelper registers the observer on init and removes it on deinit.

To test:

• Verify the media uploads function correctly (compare with the production version)

Regression Notes

1. Potential unintended areas of impact: Media Uploads
2. What I did to test those areas of impact (or what existing automated tests I relied on): n/a
3. What automated tests I added (or what prevented me from doing so): n/a

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.

UI Changes testing checklist:

• 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)

[kean]

I think it meant to be mediaObserverReceipt == nil.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

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

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	Build Number	pr21352-ce6895d
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	Commit	ce6895d
	App Center Build	jetpack-installable-builds #5836

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[momo-ozawa]

Nice catch! Can you update the release notes to capture this change? Thanks!

[kean]

Nice catch! Can you update the release notes to capture this change? Thanks!

I update them right before the merge to avoid merge conflicts 😅