fix: enforce uniqueItems on generated array fields

[XiaolongZhang-TT]

Description

datamodel-code-generator drops uniqueItems, so generated list fields accept duplicate entries in violation of the schema. Three UCP array properties declare uniqueItems: true at the 2026-04-08 release this SDK targets: context.eligibility, card_payment_instrument.brands, and identity_linking.required_claims.

This extends postprocess_models.py to collect array property names declared with uniqueItems and inject a field_validator(mode="after") into each generated class that declares a matching list field. The check uses equality (item in seen) rather than a set, so it holds for both hashable (str) and unhashable (Pydantic model) items. It mirrors the data-driven, idempotent approach already used for minProperties (#55), and complements the open contains work in #57 — the third constraint the generator drops.

Two generated models carry the affected list fields and gain the validator: Context.eligibility and Constraints.brands. required_claims has no generated typed field (ScopePolicy is extra="allow" free-form, so the field is an untyped extra key), so it is not enforceable and is skipped. Verified against the real release/2026-04-08 schemas that exactly these two models are touched on a fresh tree.

Note: like #57, this modifies postprocess_models.py, so it may need a rebase if #57 lands first.

Category (Required)

Please select one or more categories that apply to this change.

• 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

None. (Follow-up to the constraint-enforcement work in #55; complements the open contains enforcement in #57.)

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)

The injected validator (e.g. Constraints.brands):

@field_validator("brands", mode="after")
def _enforce_unique_items_brands(cls, value):  # noqa: N805
    """JSON Schema uniqueItems: reject duplicate entries."""
    if value is None:
        return value
    seen = []
    for item in value:
        if item in seen:
            raise ValueError("Items must be unique (schema uniqueItems=true)")
        seen.append(item)
    return value

Before: Constraints(brands=["visa", "visa"]) was accepted. After: it raises ValidationError ("Items must be unique").

postprocess run reports: uniqueItems fields ['brands', 'eligibility', 'required_claims'] -> 2 module(s) patched (context.py, card_payment_instrument.py).

Full suite green as CI runs it (python -m unittest discover -s tests -p "test_*.py"): 35 passed, 0 failed. ruff (--ignore D,E501, matching the pre-commit hook), ruff format, and codespell are clean.