Fix schema validator to support published manifest metadata

[GsCommand]

Motivation

• The CI schema validation failed because manifest.json is in "publication_state": "published" with a non-null release_date, while the validator only allowed pre-publication metadata; the validator needs to be state-aware so published manifests validate correctly.

Description

• Updated scripts/validate-all.mjs to allow publication_state to be either repository-validated-not-yet-published or published and to enforce release_date === null for the former and an ISO YYYY-MM-DD string for the latter; also updated CHANGELOG.md to document the validator change.

Testing

• Ran npm ci and npm run validate (the run initially failed on the manifest assertion before the fix) and after the change ran npm test and npm run validate:schemas, all of which passed.

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Codex Task