[Tests] Cover manifest.json creation and merge behaviour

[github-actions]

WHY are these changes introduced?

createOrUpdateManifestFile in packages/app/src/cli/services/build/steps/include-assets/generate-manifest.ts had no direct test coverage. Its merge contract is what lets several build steps contribute to the same manifest.json, and its recovery path is what keeps a corrupt manifest from failing a build — but both were only reachable through include-assets-step tests that always start from an empty output directory, so the merge and recovery branches were never exercised. bundle-ui-step.test.ts mocks the module out entirely.

WHAT is this pull request doing?

Adds a co-located test file for generate-manifest.ts covering the two exported functions at their public seam, using real temporary directories:

• resolveOutputDir for both the file and directory forms of an extension outputPath.
• createOrUpdateManifestFile creating the output directory when it does not exist, merging new fields into an existing entry, replacing an entry when either side is not a plain object, and starting fresh when the existing manifest.json is not valid JSON.

Each test was checked against a deliberately broken version of the corresponding branch to confirm it fails when the behaviour regresses. No production code changed.

How to manually test your changes?

CI

Post-release steps

None.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add