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/firecracker/gateway_transport.py

@@ -0,0 +1,56 @@
+"""The `GatewayTransport` for the gateway running in the gateway microVM
+(PRD 0070).
+
+How the launcher stages files + runs commands in the running gateway: the
+docker exec/cp equivalents over SSH (dropbear + the stable infra key). The
+backend-neutral provisioning logic that drives it lives in
+`backend.docker.gateway_provision`.
+"""
+
+from __future__ import annotations
+
+import os
+import shlex
+import stat
+import subprocess
+from pathlib import Path
+
+from ...gateway import GatewayProvisionError
+from . import util
+
+
+class FirecrackerGatewayTransport:
+    """`GatewayTransport` for the gateway running in the gateway VM — the docker
+    exec/cp equivalents over SSH (dropbear + the stable infra key)."""
+
+    def __init__(self, private_key: Path, guest_ip: str) -> None:
+        self._key = private_key
+        self._ip = guest_ip
+
+    def exec(self, argv: list[str]) -> None:
+        proc = subprocess.run(
+            util.ssh_base_argv(self._key, self._ip) + [shlex.join(argv)],
+            capture_output=True, text=True, timeout=60, check=False,
+        )
+        if proc.returncode != 0:
+            raise GatewayProvisionError(
+                f"infra gateway exec {argv!r} failed: {proc.stderr.strip()}")
+
+    def cp_into(self, src: str, dest: str) -> None:
+        # Preserve the source mode (docker cp does): the access-hook is staged
+        # 0700 and git-http execs it directly — a plain `cat >` would land it
+        # 0644 and the exec fails with EACCES; keys stay 0600.
+        mode = stat.S_IMODE(os.stat(src).st_mode)
+        q = shlex.quote(dest)
+        proc = subprocess.run(
+            util.ssh_base_argv(self._key, self._ip)
+            + [f"cat > {q} && chmod {mode:o} {q}"],
+            input=Path(src).read_bytes(), capture_output=True, timeout=30, check=False,
+        )
+        if proc.returncode != 0:
+            raise GatewayProvisionError(
+                f"infra gateway cp {src} -> {dest} failed: "
+                f"{proc.stderr.decode(errors='replace').strip()}")
+
+
+__all__ = ["FirecrackerGatewayTransport"]