fix(repack): wrap babel-loader fallback value in an array - #1456

Summary

BabelPlugin previously set resolveLoader.fallback['babel-loader'] to a bare string path (e.g. /path/to/babel-loader). Rspack's resolver and downstream tooling that consumes the resolved loader config (notably RSDoctor) expects the value to be an array, so the bare string caused:

error Given napi value is not an array on NapiResolveOptions.fallback

The fix wraps the value in a single-element array. The record shape is unchanged, which still satisfies:

• Rspack's ResolveAlias = { [x: string]: string | false | (string | false)[] } | false
• Webpack's resolve loader fallback (record { [name]: string | false | string[] })

Changes

• packages/repack/src/plugins/BabelPlugin.ts — wrap require.resolve('babel-loader') in [ ] so the value is string[] rather than string.
• packages/repack/src/plugins/__tests__/BabelPlugin.test.ts — new tests covering the empty and pre-populated record paths.
• .changeset/babel-loader-resolve-loader-fallback-array.md — patch changeset for @callstack/repack.

Why

Rspack's resolver pipeline and RSDoctor's loader intercept expect the value of each resolveLoader.fallback entry to be an array. A bare string passes the TS types (which allow string | string[]) but trips Rspack's NAPI binding when downstream consumers try to use it.

Test plan

• pnpm typecheck passes for all 11 packages
• pnpm lint:ci passes
• pnpm test in packages/repack passes 314 tests (added 2 new tests for BabelPlugin)
• Manual: verify RSDoctor works in a fresh Rspack project

Closes #1455