fix(payment-actions): disambiguate action.ready result

[pranshu-10]

Description

Constrains the successful action.ready result to an empty object.

Problem

The action.ready result uses oneOf for:

1. An empty success acknowledgement.
2. An error_response.

The success branch previously accepted any JSON object. A valid error response therefore matched both branches, causing oneOf validation to fail.

Fix

Add maxProperties: 0 to the success branch so that only {} is accepted as a successful acknowledgement.

Verification

• Empty success object {} remains valid.
• Valid error_response objects now match only the error branch.
• Invalid non-empty success objects are rejected.

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

N/A — this PR fixes a directly identified schema validation bug.

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)

import copy
import json
import subprocess
from pathlib import Path

from jsonschema import Draft202012Validator
from referencing import Registry, Resource

repo = Path.cwd()
filename = "source/services/payment-actions/embedded.openrpc.json"
error_schema_id = "https://ucp.dev/schemas/common/types/error_response.json"

registry = Registry()

for path in (repo / "source/schemas").rglob("*.json"):
    document = json.loads(path.read_text())

    if "$id" in document:
        registry = registry.with_resource(
            document["$id"],
            Resource.from_contents(document),
        )

error = {
    "ucp": {
        "version": "2026-08-14",
        "status": "error",
    },
    "messages": [
        {
            "type": "error",
            "code": "payment_failed",
            "content": "Authentication failed",
            "severity": "unrecoverable",
        }
    ],
}

samples = {
    "empty success": {},
    "valid error": error,
    "unexpected object": {"unexpected": True},
}

for revision in ("HEAD^", "HEAD"):
    document = json.loads(
        subprocess.check_output(
            ["git", "show", f"{revision}:{filename}"],
            text=True,
        )
    )

    schema = copy.deepcopy(
        next(
            method["result"]["schema"]
            for method in document["methods"]
            if method["name"] == "action.ready"
        )
    )

    # Resolve the OpenRPC relative reference through the local registry.
    schema["oneOf"][1]["$ref"] = error_schema_id

    validator = Draft202012Validator(schema, registry=registry)

    print(f"\n{revision}")

    for name, value in samples.items():
        print(f"  {name}: {validator.is_valid(value)}")

Used the above python script to validate following test cases for before and after the fix

1. Valid empty success

 {}

Expected:

Before fix: PASS
After fix: PASS

2. Valid error response

  {
    "ucp": {
      "version": "2026-08-14",
      "status": "error"
    },
    "messages": [
      {
        "type": "error",
        "code": "payment_failed",
        "content": "Authentication failed",
        "severity": "unrecoverable"
      }
    ]
  }

Expected:

Before fix: FAIL incorrectly
After fix: PASS

3. Invalid non-empty acknowledgement

  {
    "unexpected": true
  }

Expected:

Before fix: PASS incorrectly
After fix: FAIL