Fix strict transform array element type

[Boulea7]

Summary:

Fixes #53899.

MergeUnion made every transform key optional before MaximumOneOf selected a key. Indexing that optional mapped type added undefined to each element of ViewStyle['transform'] under strict null checks. This makes the intermediate keys required, preserving the existing one-key transform constraint while keeping undefined out of the array element type.

Changelog:

[General] [Fixed] - Prevent strict transform array element types from including undefined

Test Plan:

Validated against main at d7ff82ebe4ed5155b955fd60b31e02bbc76a3c66:

• yarn build-types and yarn build-types --validate — passed; the snapshot was regenerated from Flow and is up to date.
• yarn test-generated-typescript and yarn test-typescript-legacy — passed.
• yarn test scripts/js-api/build-types --runInBand --ci --no-cache — 21 suites, 149 tests, and 131 snapshots passed.
• rn_flow_tmp=$(mktemp -d) && yarn flow-check --max-workers 8 --temp-dir "$rn_flow_tmp" — passed; this is a copyable equivalent of the isolated temporary-directory invocation used for validation.
• ./node_modules/.bin/prettier --check packages/react-native/Libraries/StyleSheet/private/_TransformStyle.js packages/react-native/ReactNativeApi.d.ts — passed.
• ./node_modules/.bin/eslint --max-warnings 0 packages/react-native/Libraries/StyleSheet/private/_TransformStyle.js — passed.
• git diff --check d7ff82ebe4ed5155b955fd60b31e02bbc76a3c66 -- — passed.

The generated and legacy TypeScript suites use strict: false, so they do not exercise this strict-null regression. After generating types, the following standalone command reproduces the public-type check from the repository root. It creates a temporary probe inside the checkout so react-native resolves to the generated workspace types, then removes only that file and its directory even when the check fails:

(
  set -e
  rn_probe_dir=$(mktemp -d ./.rn-transform-probe.XXXXXX)
  trap 'rm -f -- "$rn_probe_dir/transform-probe.ts"; rmdir -- "$rn_probe_dir"' EXIT
  cat > "$rn_probe_dir/transform-probe.ts" <<'TS'
import type {ViewStyle} from 'react-native';

function transformKeys(transform: NonNullable<ViewStyle['transform']>) {
  if (typeof transform === 'string') {
    return [];
  }
  return transform.map(element => Object.keys(element));
}

void transformKeys;
TS
  ./node_modules/.bin/tsc --noEmit --strictNullChecks --skipLibCheck --moduleResolution bundler --module esnext --target es2020 --jsx react --types react "$rn_probe_dir/transform-probe.ts"
)

On unmodified main, this probe fails with TS2769 at Object.keys(element) because the array element includes undefined. The same probe passes after the fix and regeneration. Additional temporary checks passed for single-key objects, readonly arrays, animated values, string transforms, an omitted transform, and the existing optional selected property; they also confirm rejection of undefined array elements and objects with multiple transform keys. Those boundary checks are not part of the minimal command above.

No additional test script or strict TypeScript configuration is included in this PR, as requested in review.

The full repository lint and Jest suites, native builds, and RNTester were not run for this type-only change.

Codex was used for this update and its validation.

[github-actions]

Warning

JavaScript API change detected

This PR commits an update to ReactNativeApi.d.ts, indicating a change to React Native's public JavaScript API.

• Please include a clear changelog message.
• This change will be subject to additional review.

This change was flagged as: POTENTIALLY_BREAKING

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