Simplify not checks on duration designator schemas

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: Refactors the ISO 8601 duration “designator” schemas to consolidate disallowed bare designators.

Changes:

• In designator.json, replaced multiple allOf + not + const checks with a single not + enum.
• In designator-extension.json, applied the same simplification while also covering the negative variants.

Technical Notes: This should be behavior-preserving: the schema still rejects P/PT (and -P/-PT in the extension) while validating the allowed pattern-based duration formats.

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[augmentcode]

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