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>

bot_bottle/backend/macos_container/gateway.py

@@ -184,10 +184,10 @@ class MacosGateway(Gateway):
     def provisioning_transport(self) -> GatewayTransport:
         """The exec/cp transport git-gate provisioning stages per-bottle repos +
         deploy keys through (over the `container` CLI)."""
-        # Local import: gateway_provision imports GATEWAY_NAME from this module,
-        # so importing AppleGatewayTransport at module scope would cycle.
-        from .gateway_provision import AppleGatewayTransport
-        return AppleGatewayTransport(self.name)
+        # Local import: gateway_transport imports GATEWAY_NAME from this module,
+        # so importing MacosGatewayTransport at module scope would cycle.
+        from .gateway_transport import MacosGatewayTransport
+        return MacosGatewayTransport(self.name)
 
 
 __all__ = [