feat: Handle remote editor load failure

[dcalhoun]

Description

Gracefully handle failed attempts to load a site's remote editor by falling back to the local, default editor and displaying a notice to the user.

Related:

• https://github.com/wordpress-mobile/GutenbergKit/pull/27
• https://github.com/Automattic/dotcom-forge/issues/9085

To test: See https://github.com/wordpress-mobile/GutenbergKit/pull/27.

Regression Notes

1. Potential unintended areas of impact None, should be limited to the experimental editor.
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) None, we are not focused on automating tests for this experimental feature.

PR submission checklist:

• [x] I have completed the Regression Notes.
• [x] I have considered adding unit tests for my changes.
• [x] I have considered adding accessibility improvements for my changes.
• [x] 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]

	2 Warnings
:warning:	Modules/Package.swift was changed without updating its corresponding Package.resolved. Please resolve the Swift packages in Xcode.
:warning:	PR is not assigned to a milestone.

Generated by :no_entry_sign: Danger

[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	pr23624-eebbbfa
	Version	25.4
	Bundle ID	com.jetpack.alpha
	Commit	eebbbfa22129bf4c4388e616a3adae0e0f2d2f21
	App Center Build	jetpack-installable-builds #9881

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[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	pr23624-eebbbfa
	Version	25.4
	Bundle ID	org.wordpress.alpha
	Commit	eebbbfa22129bf4c4388e616a3adae0e0f2d2f21
	App Center Build	WPiOS - One-Offs #10839

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.