build(model-server): don't include unused openapi-generated files

We don't use the DefaultApi.kt generated for every API at the moment. No
need to compile those.

Author: languitar

model-server/.openapi-generator-ignore

@@ -0,0 +1 @@
+**/DefaultApi.kt

model-server/build.gradle.kts

@@ -279,6 +279,7 @@ openApiFiles.forEach {
                 "supportingFiles" to "Paths.kt",
             ),
         )
+        ignoreFileOverride.set("${layout.projectDirectory.file(".openapi-generator-ignore")}")
     }
 
     // Ensure that the OpenAPI generator runs before starting to compile