[Refactor] Deduplicate test helpers

[samzong]

Summary

• Add shared JSON golden helpers in internal/testutil for formatted golden assertions and sorted rawir.RawModule output.
• Use those helpers in backend and normalize golden tests.
• Add local render test fixture helpers for temporary Go module setup and generated output reads.

Verification

• python3 /Users/x/.agents/skills/codex-review/scripts/run_codex_review.py --uncommitted
• make check

Compatibility

None. This is test-only cleanup and does not change generated command shape, catalog schema, auth/body/output behavior, or docs.

Checklist

• Tests or focused verification cover the changed surface.
• No user-facing behavior changes.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.