Fix content collection validation errors showing raw JSON

[moktamd]

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.

[changeset-bot]

🦋 Changeset detected

Latest commit: ccdceca

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks

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_{Comparing moktamd:fix/zod-v4-validation-error-format (ccdceca) with main (2dca307)¹}

Footnotes

1. No successful run was found on main (efcd607) during the generation of this report, so 2dca307 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩