fix: enforce dependent required fields

[FanouZeng-TT]

Description

time_interval.json uses dependentRequired to require opens and closes together, but datamodel-code-generator drops that keyword. The generated TimeInterval therefore accepted either field alone, and inherited models such as ExceptionHour accepted the same invalid shape.

Fix: scan root object schemas for valid dependentRequired maps and inject a property-presence validator into matching generated models. The check uses model_fields_set plus extra keys, so explicit null still counts as a provided property. Request variants whose projection removes either side of a rule are left unchanged.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

• ./generate_models.sh 2026-08-25 (second run byte-identical)
• Python 3.10, 3.11, and 3.12: 148 tests passed, 4 skipped
• full pre-commit suite passed
• git diff --check passed