Fixes #15976

Since the Zod v3 → v4 migration (#14956), $ZodError.message returns a JSON-stringified array instead of a human-readable string. The three content error definitions in errors-data.ts used error.message directly, producing output like:

  **: [
  {
    "expected": "object",
    "code": "invalid_type",
    ...
  }
]

This iterates over error.issues instead, producing:

  **label**: Required

This matches the pattern already used in content/loaders/errors.ts and content/utils.ts.