Editor fails to load when a cached capability flag is stale

[dcalhoun]

Description

When a host caches a site capability (shouldUseThemeStyles/themeStyles, shouldUsePlugins/plugins) and the site later loses the backing REST route, the editor fails to load with an error instead of degrading to the default experience.

The 404 is a definitive, correct answer — the route does not exist — and it's the same answer route discovery would have given had it re-run. Both platforms already have a supported "capability absent" value (EditorSettings.undefined, LocalEditorAssetManifest.empty), so this should resolve to that value rather than surfacing as an error.

The user-facing message is also WordPress's raw REST string, which is a separate problem affecting every editor load failure.

Steps to reproduce

1. Activate the Gutenberg plugin on a test site
2. Open the site's Editor Configuration in the Demo app — Supports Theme Styles should be ✅
3. Deactivate the Gutenberg plugin
4. Open the editor

Expected: editor loads with default editor styles.
Actual: an error view.

iOS: Editor Error — No route was found matching the URL and request method.
Android: Failed to load editor — rest_no_route: No route was found matching the URL and request method.

iOS	Android

Cause

Identical on both platforms:

1. Host caches the capability as true; the site changes underneath it without re-running discovery
2. fetchEditorSettings no longer short-circuits and issues the request — iOS RESTAPIRepository.swift#L93-L97, Android RESTAPIRepository.kt#L88-L91
3. WordPress returns 404 rest_no_route; the client throws a WP-error type
4. Nothing treats it as recoverable, so it propagates out of prepare()
5. The library's own error view renders the exception message

Proposed fix

Treat 404 rest_no_route on an optional dependency as the capability being absent:

• iOS: RESTAPIRepository.fetchEditorSettings() → .undefined
• iOS: EditorAssetLibrary.fetchManifest() → LocalEditorAssetManifest.empty
• Android: RESTAPIRepository.fetchEditorSettings() → EditorSettings.undefined
• Android: EditorAssetsLibrary.fetchManifest() → LocalEditorAssetManifest.empty

Each of these already returns exactly that value when its capability flag is false — iOS settings, iOS manifest, Android settings, Android manifest. The fix extends the same outcome to the case where the endpoint itself reports the capability absent.

Scope the guard to rest_no_route specifically so genuine 404s elsewhere (e.g. a missing post) stay fatal.

This must not respect any network-fallback policy. A 404 is a successful round-trip carrying an authoritative answer; reachability policy is irrelevant. Gating it would leave hosts with fallback disabled (and all Android hosts) hitting this same wall.

Notes

• Not demo-only. WP-Android's EditorCapabilityResolver persists capabilities across sessions — a wider staleness window than the iOS demo's per-session one. WP-iOS is likewise affected.
• Applies equally to /wpcom/v2/editor-assets and the plugins capability.
• The iOS demo's offline config deliberately sets both capabilities true optimistically (SitePreparationView.swift#L334-L342, from feat: improve iOS offline editor experience #366). That's safe while offline, but if connectivity returns mid-session the flags are stale in exactly this way. This fix makes that optimism safe unconditionally.
• Impact data: wpios_gutenberg_kit_editor_load_failed / jpios_gutenberg_kit_editor_load_failed run ~5–20/day sustained. Segment by wordpress_error_code = rest_no_route to size this specific path. iOS-only — WP-Android has no equivalent instrumentation, so real impact is undercounted.
• Surfaced while reviewing refactor: vendor the default editor styles stylesheet #600; unrelated to those changes.