fix(editor): translate the patterns count and fall back for new editor strings

[dcalhoun]

Description

Adopts GutenbergKit#569, which localizes the block inserter's pattern count and lets host apps fall back to the editor's own strings for keys they do not translate.

Two changes here:

Translate patternsCount. GutenbergKit#569 adds EditorLocalizableString.patternsCount(Int) for a string that previously rendered as hardcoded English. Singular and plural are split so each reads naturally rather than forcing "1 pattern(s)".

Decline unhandled keys. getLocalizedString(for:) switched over the enum exhaustively, so every string the editor added broke this build until someone wrote a translation — and the break surfaced on the dependency bump, not when the string was added. Unhandled keys now return nil, and the editor supplies its own string:

@unknown default:
    nil

New editor strings render in English until translated, instead of failing to compile. @unknown default rather than a plain default because this switch currently covers every case, and a plain one warns "default will never be executed."

EditorLocalization.localize now returns String?, so getLocalizedString(for:) and EditorLocalizableString.localized return optionals to match. The install site in PostGBKEditorViewController is unchanged — { $0.localized } already fits the new closure type.

Note that @unknown default covers only keys added by a future GutenbergKit. Skipping a case that exists today is still a compile error, so this cannot silently drop a translation we meant to write.

Note

Modules/Package.swift pins a GutenbergKit revision while that PR is open. The TODO there marks restoring a version pin once it ships. This PR should not merge before GutenbergKit#569.

Testing instructions

The pattern count is the visible change:

1. Open the editor and tap + for the block inserter.
2. Switch to the Patterns tab on a site that has patterns.
3. Confirm section headers show a localized count, and that a category with exactly one pattern reads "1 pattern" rather than "1 patterns".

To verify the fallback, which has no visible effect until GutenbergKit adds a string:

1. Comment out a case in getLocalizedString(for:) — .patterns or .search render unconditionally and are easy to spot. Return nil for it instead, since removing the case outright is a compile error.
2. Run the app and open the block inserter.
3. Confirm the string renders in English rather than crashing or rendering empty.
4. Confirm the console shows [GutenbergKit:localization] Missing host translation for …, or filter Console.app on subsystem GutenbergKit. Each key reports once per launch, so scrolling a list will not repeat it.
5. Restore the case.

Nothing reports with every case handled, which is the expected state on trunk.

[wpmobilebot]

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33532
	Version	PR #25848
	Bundle ID	org.wordpress.alpha
	Commit	1311278
	Installation URL	76l3qa5e29220

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

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33532
	Version	PR #25848
	Bundle ID	com.jetpack.alpha
	Commit	1311278
	Installation URL	66jtjo56q386g

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