refactor(js): harden native inserter pattern payload shape (#467)

* refactor(js): extract native inserter pattern formatters

Pulls the inline pattern/pattern-category serialization in
`NativeBlockInserterButton` into named helpers in `src/utils/blocks.js`
and flips array fields (`blockTypes`, `categories`, `keywords`) from
`?? null` to `?? []` so the JSON shape matches the non-nullable
`List<String>` contract in the Kotlin `BlockPattern` model. Emitting
`null` for those fields would throw `SerializationException` on the
Android side.

* test(js): validate native inserter payload against JSON Schema

Adds `schemas/block-inserter-payload.schema.json` mirroring the Kotlin
`BlockInserterPayload` contract at
android/Gutenberg/src/main/java/org/wordpress/gutenberg/model/BlockInserter.kt,
plus `src/utils/blocks.test.js` which validates the output of
`preprocessBlockTypesForNativeInserter`, `formatPatternsForNativeInserter`,
and `formatPatternCategoriesForNativeInserter` against it via `ajv`.

The schema is draft-07 with `additionalProperties: false` on every
definition, and is co-located at the repo root so it can be consumed by
cross-language tooling (`quicktype`, `kotlinx-serialization` codegen)
alongside the hand-written Kotlin model. Upstream Gutenberg ships no
JSON Schema for these shapes (only JSDoc typedefs on
`WPEditorInserterItem` / `WPBlockType`), so the schema is hand-written
and tracks the Kotlin model as the source of truth.

Catches the null-vs-array drift that motivated #465, plus drift in
required fields, types, and unknown-field leaks that Kotlin's
`ignoreUnknownKeys = true` would otherwise silently tolerate.

Adds `ajv` as a direct devDependency (was transitive).

Author: jkmassel

package-lock.json

@@ -97,6 +97,7 @@
 		"@wordpress/env": "^11.2.0",
 		"@wordpress/eslint-plugin": "^24.3.0",
 		"@wordpress/prettier-config": "^4.43.0",
+		"ajv": "^8.18.0",
 		"eslint": "^8.57",
 		"eslint-import-resolver-typescript": "^4.4.4",
 		"eslint-plugin-react-refresh": "^0.4.26",