Conformance: add OpenAPI property-based fuzz tests

[mark-e-deyoung]

Summary

Add a conformance stage that runs property-based API tests generated from the OpenAPI document.

Scope

• Add schemathesis as a pinned test dependency
• Add deterministic profile for CI (seeded, bounded examples)
• Exercise authenticated and unauthenticated endpoints with explicit allow/deny map
• Export failing seeds/artifacts for reproducibility

Acceptance Criteria

• CI job runs on PRs with bounded runtime
• Failures include reproducible seed and endpoint
• Existing API contract tests remain green