message: Refactor `testCodegenContext`
  Instead of fabricating a new service shape when the caller does not
  explicitly provide one, first attempt to obtain one from the passed in
  model, which is a more faithful test code generation context.
diff: @@ -80,7 +80,9 @@ fun testCodegenContext(
  model,
  testSymbolProvider(model),
  TestRuntimeConfig,
  - serviceShape ?: ServiceShape.builder().version("test").id("test#Service").build(),
  + serviceShape
  + ?: model.serviceShapes.firstOrNull()
  + ?: ServiceShape.builder().version("test").id("test#Service").build(),
  ShapeId.from("test#Protocol"),
  settings, mode
  )
  