feat: mock transport instead of provider

[DaniPopes]

MockProvider currently tries to mock every response at the json level; this is way easier to implement and maintain at the transport level. This also allows testing with any layer and filler combination.

[yash-atreya]

MockProvider impl was for providing a smol UX improvement:

provider.asserter();
provider.push_response();
provider.push_error();

But this didn't work with layers, and made maintenance cumbersome.

OK with moving to transport impl

lgtm!