fix: enforce minProperties in generated schemas

[FanouZeng-TT]

Description

inject-schema-constraints.mjs restores JSON Schema constraints that quicktype's
typescript-zod target omits, but it did not handle minProperties. The pinned
UCP schema contains two affected shapes:

// common/types/description.json
{
  "properties": { "plain": {}, "html": {}, "markdown": {} },
  "minProperties": 1
}

// shopping/types/available_payment_instrument.json
{
  "constraints": {
    "type": "object",
    "additionalProperties": true,
    "minProperties": 1
  }
}

The generated schemas therefore accepted both an empty Description and an
empty, present constraints record:

DescriptionSchema.parse({});
AvailablePaymentInstrumentSchema.parse({ type: "card", constraints: {} });

Fix: index object-level and record-level minProperties constraints and
append a Zod refinement during generation. Object schemas retain additional
properties before counting them, matching JSON Schema's default behavior.
Regression tests cover rejection of empty objects, acceptance of non-empty
objects, optional constraints, and retention of additional description keys.

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)

• npm test — 50 tests passed
• npm run build:noEmit — passed
• npm run build — passed
• pre-commit run --all-files — passed
• Regeneration from pinned UCP release/2026-04-08 produced the committed src/spec_generated.ts byte-for-byte after formatting.