Validate language-model mock output against the schema in OpenApiMockResponsePlugin #2039

OpenApiMockResponsePlugin (#2036) can generate mock bodies with a language model, but it checks only the top-level JSON type of the model's output. If the output breaks schema constraints (e.g. minimum, required, enum, pattern), it is still served as the mock.

Proposal: once OpenApiValidationPlugin (#2037) is merged, validate the model's output with its OpenApiSchemaValidator. If validation fails, fall back to the schema-based sample generator (and log why).

On hold until these are merged:

• Add OpenApiMockResponsePlugin to mock APIs from OpenAPI specs #2036 (OpenApiMockResponsePlugin, closes Mock API by pointing to Open API spec #531)
• Add OpenApiValidationPlugin to validate API traffic against local OpenAPI specs #2037 (OpenApiValidationPlugin, closes Check if the API matches its local OpenAPI spec #779)