New sample: mock an API from its OpenAPI spec using OpenApiMockResponsePlugin #73

Create a sample that shows how to mock an API with Dev Proxy's new OpenApiMockResponsePlugin (dotnet/dev-proxy#2036, closes dotnet/dev-proxy#531).

The sample should include:

• An OpenAPI spec for a sample API
• A Dev Proxy config that uses OpenApiMockResponsePlugin with specFile pointing to the spec
• Optional: a languageModel configuration that uses a local language model (e.g. Ollama) to generate realistic responses, falling back to schema-based values when no model is available
• A README that explains how the sample works and how to use it