fix: enforce conditional required fields in generated models

[FanouZeng-TT]

Description

pagination.json requires cursor when has_next_page is true, but datamodel-code-generator drops the schema's if/then rule and emits cursor as unconditionally optional. Response(has_next_page=True) therefore passed validation without a continuation cursor.

Fix: extract simple single-discriminator const/enum conditions with then.required and inject an idempotent Pydantic model validator. Required-field presence uses model_fields_set, preserving JSON Schema's key-presence semantics; complex conditions are skipped.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure
• Maintenance
• SDK
• Samples / Conformance
• UCP Schema
• Community Health (.github)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove the fix is effective.
• New and existing unit tests pass locally with my changes.
• I have included/updated the relevant JSON schemas (Core/Capability only).
• I have regenerated Python Pydantic models by running generate_models.sh 2026-04-08.

Screenshots / Logs (if applicable)

• Python unit tests: 61 passed
• pre-commit run --all-files: passed
• pinned regeneration: generated pagination model matches byte-for-byte