fix: handle transition-shaped ucp_request markers in eval_prop_inclusion

[FanouZeng-TT]

Description

eval_prop_inclusion in preprocess_schemas.py did not handle the
transition-shaped ucp_request marker that UCP introduced for
cart.json's id field:

"ucp_request": {
  "update": {
    "transition": {
      "from": "required",
      "to": "omit"
    }
  }
}

When marker.get(op) returns a dict (the {"transition": {...}} form), the
existing branches only compare against the strings "omit" / "required" /
"optional", none of which match a dict. The field therefore kept its base
default of include=True and is_required = name in base_required.

Result: the generated CartUpdateRequest marks id: str as required, but the
spec says id should be omitted in update requests (prefer transport-level
identification, no duplication). A field that a string "omit" marker handles
everywhere else is silently treated as required under the new transition form.

Fix: when the operation marker resolves to a dict, unwrap its
transition.to value before the existing "omit" / "required" /
"optional" checks, so transition markers behave like their plain-string
equivalents.

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)

N/A — codegen pipeline fix, no schema/field removal.

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[damaz91]

@oufanz thanks for opening this PR. Can you please sign the CLA at https://cla.developers.google.com/ so we can accept your contributions? Thanks

[FanouZeng-TT]

Thanks @damaz91! The CLA has been signed now — the cla/google check is passing (SUCCESS). Appreciate the quick review, and happy to address any feedback on the change.