test: raise coverage with focused unit tests

[zckjzfy]

Summary

• Add focused tests for config helper error paths, extension/project behavior, loaders, and website config/components.
• Keeps the change test-only while raising all-files line coverage above the task target.

Coverage

• Baseline all-files lines: 48.8%
• Updated all-files lines: 86.77%
• Updated src lines: 88.1%

Validation

• pnpm exec vitest run
• pnpm exec vitest run --coverage
• pnpm run lint (warnings only from existing files)