refactor(gateway): each backend's transport in its own gateway_transport.py

Give every backend a `gateway_transport.py` holding just its `GatewayTransport`
impl, named consistently with the backend: `DockerGatewayTransport`,
`MacosGatewayTransport` (was `AppleGatewayTransport`), and
`FirecrackerGatewayTransport` (was `SshGatewayTransport`).

- docker: split `DockerGatewayTransport` out of `gateway_provision.py`, which
  keeps only the backend-neutral provisioning logic (provision_git_gate /
  deprovision_git_gate) the other backends share.
- macOS: `gateway_provision.py` held only the transport, so it's replaced by
  `gateway_transport.py`.
- firecracker: move the transport out of `gateway.py`.

Behaviour-preserving; importers + tests updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_macos_gateway.py

@@ -135,11 +135,11 @@ class TestMacosGatewayLifecycle(unittest.TestCase):
                 self.gw.ca_cert_pem(timeout=0)
 
     def test_provisioning_transport_targets_the_gateway_container(self) -> None:
-        from bot_bottle.backend.macos_container.gateway_provision import (
-            AppleGatewayTransport,
+        from bot_bottle.backend.macos_container.gateway_transport import (
+            MacosGatewayTransport,
         )
         transport = self.gw.provisioning_transport()
-        assert isinstance(transport, AppleGatewayTransport)
+        assert isinstance(transport, MacosGatewayTransport)
         self.assertEqual(self.gw.name, transport.gateway)