fix(rest/python): publish the REST OpenAPI schema URL

[FanouZeng-TT]

Description

The Python REST sample advertises this service schema in its live discovery profile:

"schema": "https://ucp.dev/2026-04-08/services/shopping/openapi.json"

That URL returns HTTP 404. The published v2026-04-08 file is
services/shopping/rest.openapi.json, which returns HTTP 200 and matches the
REST service schema name in the UCP source tree.

Fix: point the Python discovery profile to the published REST OpenAPI URL
and add an integration assertion over the /.well-known/ucp response.

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 — verified the old URL returns HTTP 404 and the corrected URL returns HTTP 200. The targeted discovery profile tests (3/3), all 27 integration tests when applied with the already approved PR #196 baseline fix, and all repository pre-commit hooks pass locally. The current main alone has 14 payment-model mismatch errors addressed by #196; this change adds no failures and does not conflict with it.