feat(android): add block inserter payload model

[jkmassel]

Summary

Groundwork for the native block inserter feature on Android: @Serializable Kotlin data classes mirroring the JSON shape produced by BlockInserterBridge in src/components/native-inserter/index.jsx. Parser tests cover full payload, minimal payload, fallback category, explicit nulls on nullable strings, null arrays on pattern fields, and malformed input.

Extracted from #463 so each PR stays narrow. The original #463 mixed this model with a separate JS bridge fix, and feedback on the stack prompted a split.

Scope

• BlockInserter.kt — BlockInserterPayload, BlockInserterSection, BlockType, BlockPattern, BlockPatternCategory, SourceRect.
• BlockInserterTest.kt — five parser tests.

Not referenced from production code yet; the Android @JavascriptInterface receiver lands in a later PR in the stack.

A note on strictness

The three List<String> fields on BlockPattern — blockTypes, categories, keywords — are non-nullable with = emptyList() defaults. kotlinx-serialization's default behavior applies the default when the JSON key is missing, but throws SerializationException when the key is present and explicitly null. The current JSX emits ?? null for these fields; that matching JS fix (?? []) lands in a follow-up PR on the stack.

The rejects null arrays on pattern fields test pins this contract so the parser can't be silently relaxed later without a conscious decision. If the tradeoff ever needs to flip (e.g. to coerceInputValues = true), that test fails and the decision happens in review.

Test plan

• ./gradlew :Gutenberg:testDebugUnitTest detekt :Gutenberg:lintDebug — passes locally

Related

• feat(android): add block inserter payload model + JS dispatch fix #463 — the original combined PR this was split out of (will be rescoped to JS-only)