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/docker/gateway_provision.py

@@ -16,7 +16,6 @@ from __future__ import annotations
 
 import re
 
-from .util import run_docker
 from ...git_gate import GitGatePlan, git_gate_render_provision
 from ...gateway import GatewayProvisionError, GatewayTransport
 
@@ -27,27 +26,6 @@ from ...gateway import GatewayProvisionError, GatewayTransport
 _SAFE_BOTTLE_ID = re.compile(r"[A-Za-z0-9_-]+")
 
 
-class DockerGatewayTransport:
-    """`GatewayTransport` for the docker gateway container (exec/cp)."""
-
-    def __init__(self, gateway: str) -> None:
-        self.gateway = gateway
-
-    def exec(self, argv: list[str]) -> None:
-        proc = run_docker(["docker", "exec", self.gateway, *argv])
-        if proc.returncode != 0:
-            raise GatewayProvisionError(
-                f"gateway exec {argv!r} failed: {proc.stderr.strip()}"
-            )
-
-    def cp_into(self, src: str, dest: str) -> None:
-        proc = run_docker(["docker", "cp", src, f"{self.gateway}:{dest}"])
-        if proc.returncode != 0:
-            raise GatewayProvisionError(
-                f"gateway cp {src} -> {dest} failed: {proc.stderr.strip()}"
-            )
-
-
 def _require_safe(bottle_id: str) -> None:
     if not _SAFE_BOTTLE_ID.fullmatch(bottle_id):
         raise GatewayProvisionError(f"unsafe bottle id {bottle_id!r}")
@@ -109,5 +87,5 @@ def deprovision_git_gate(transport: GatewayTransport, bottle_id: str) -> None:
 
 __all__ = [
     "provision_git_gate", "deprovision_git_gate",
-    "GatewayProvisionError", "GatewayTransport", "DockerGatewayTransport",
+    "GatewayProvisionError", "GatewayTransport",
 ]