fix: disable response model for mock webhook route

[FanouZeng-TT]

Description

MockWebhookServer now returns either a JSONResponse for a simulated transient failure or a dictionary for success:

async def order_event(...) -> JSONResponse | dict[str, str]:

FastAPI treats that union annotation as a response model and raises FastAPIError while registering the route because JSONResponse is not a valid Pydantic response field. As a result, all nine webhook tests fail during setup before starting the mock server.

Fix: set response_model=None on the mock webhook route. This disables response-schema generation for the test endpoint while preserving its existing 500/200 status codes, bodies, and event recording.

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)

• Verified the mock route still returns 500 {"status":"retry"} followed by 200 {"status":"ok"} and records both statuses.
• webhook_structure_test.py: 6/6 passed.
• webhook_test.py: 3/3 passed.
• All 17 conformance test files passed against the reference server.
• Repository-wide pre-commit and git diff --check passed.