fix: keep additional properties on open schemas

[sakinaroufid]

Description

Generated objects are bare z.objects, which strip unknown keys, so objects the spec declares open (additionalProperties: true) lose data on parse. CheckoutCompleteRequestSchema.parse(body) drops payment.instruments[].credential.token (the credential is the open payment_credential.json) and ap2, and checkouts and carts lose extension fields such as fulfillment and discounts. The getting-started guide on ucp.dev works with the parsed .data, so code that follows it is affected.

• Commit 1: a new generation step, scripts/retain-additional-properties.mjs, appends .catchall(z.any()) to a generated object when every source schema with its property set is open, following $ref and allOf across the authored, projected and discovery trees. 71 objects gain it. Six that also stand for a closed or undeclared schema are left as generated, and the step reports them.
• Commit 2: GetProductResponseSchema dropped available and exists. quicktype's allOf intersection of two arrays keeps only one side's items, so detail_product's redeclared options[].values[] lost to product.json's. The projection now moves such a redeclaration into a trailing allOf branch, the side quicktype keeps.

Compatibility: on those 71 objects, parse now keeps unknown keys, and their z.infer types gain [k: string]: any, so an unmodelled property is no longer a type error. Closed schemas still strip.

Category (Required)

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

Related Issues

None.

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable). (n/a)
• 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. (n/a)
• I have regenerated Python Pydantic models. (n/a; Zod models regenerated against release/2026-08-25 and match byte for byte)

Screenshots / Logs (if applicable)

credential.token after parse    before: dropped    after: kept
spec example payloads losing data on parse: 57 before, 6 after (none involve payment credentials)
npm test: 282 pass, 0 fail      npm run build: OK