[Vertex AI] Replace ModelContent.Part enum with protocol/structs (#13767)

FirebaseVertexAI.podspec

@@ -62,6 +62,7 @@ Firebase SDK.
     ]
     unit_tests.resources = [
       unit_tests_dir + 'vertexai-sdk-test-data/mock-responses/**/*.{txt,json}',
+      unit_tests_dir + 'Resources/**/*',
     ]
   end
 end