fix(plugin-bff): preserve package manifest trailing newline

[omarshibli]

Summary

Preserve a trailing newline when the BFF client generator rewrites package.json.

Also await the package manifest update so callers do not race the generated manifest write.

Problem

plugin-bff rewrites package.json with JSON.stringify(packageJson, null, 2) and no trailing newline. In repositories that normalize manifests during dependency install, this makes framework generation and package manager normalization fight each other.

Changes

• Append \n when writing the generated package manifest.
• Await setPackage from clientGenerator.
• Add regression coverage for generated package manifests.

Validation

• pnpm --filter @modern-js/plugin-bff test
• pnpm exec biome check packages/cli/plugin-bff/src/utils/clientGenerator.ts packages/cli/plugin-bff/tests/clientGenerator.test.ts

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