Validate incoming OpenAPI docs using OpenAPIKit's built-in validation

Sources/_OpenAPIGeneratorCore/GeneratorPipeline.swift

@@ -122,7 +122,39 @@ func makeGeneratorPipeline(
                     diagnostics: diagnostics
                 )
             },
-            postTransitionHooks: []
+            postTransitionHooks: [
+                { doc in
+
+                    // Run OpenAPIKit's built-in validation.
+                    try doc.validate()
+
+                    // Validate that the document is dereferenceable, which
+                    // catches reference cycles, which we don't yet support.
+                    _ = try doc.locallyDereferenced()
+
+                    // Also explicitly dereference the parts of components
+                    // that the generator uses. `locallyDereferenced()` above
+                    // only dereferences paths/operations, but not components.
+                    let components = doc.components
+                    try components.schemas.forEach { schema in
+                        _ = try schema.value.dereferenced(in: components)
+                    }
+                    try components.parameters.forEach { schema in
+                        _ = try schema.value.dereferenced(in: components)
+                    }
+                    try components.headers.forEach { schema in
+                        _ = try schema.value.dereferenced(in: components)
+                    }
+                    try components.requestBodies.forEach { schema in
+                        _ = try schema.value.dereferenced(in: components)
+                    }
+                    try components.responses.forEach { schema in
+                        _ = try schema.value.dereferenced(in: components)
+                    }
+
+                    return doc
+                }
+            ]
         ),
         translateOpenAPIToStructuredSwiftStage: .init(
             preTransitionHooks: [],