fix(macos-container): launch gateway as a module, not /app/gateway_init.py

The macOS infra container's init script still spawned the supervisor as
`python3 /app/gateway_init.py`. 5ad3449 moved bot_bottle from flat files
under /app into a pip-installed package, so that file no longer exists:
the supervisor never started, mitmdump never generated its CA, and the
launch failed downstream with a misleading "gateway CA not available"
error pointing at TLS rather than at the supervisor.

This is the same defect 127ba49 fixed for the firecracker backend. It
survived on macOS because CI has no Apple Container runner — the KVM
runner added in c193b04 covers firecracker, and ubuntu-latest covers
docker, but nothing exercises macos_container.

test_macos_infra asserted the substring "gateway_init.py", which the
broken path satisfied; it now asserts the module form, matching the
assertion 127ba49 already made in test_firecracker_infra_vm.

Verified end to end on the macos-container backend: image builds, CA is
generated, agent registers, and a headless bottle runs a prompt.

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

bot_bottle/backend/macos_container/infra.py

@@ -101,7 +101,7 @@ def _init_script(port: int) -> str:
         # Gateway data plane, multi-tenant against the local control plane.
         f"( cd /app && BOT_BOTTLE_GATEWAY_DAEMONS={_GATEWAY_DAEMONS} "
         f"BOT_BOTTLE_ORCHESTRATOR_URL=http://127.0.0.1:{port} "
-        f"SUPERVISE_DB_PATH={_DB_PATH_IN_CONTAINER} python3 /app/gateway_init.py ) &\n"
+        f"SUPERVISE_DB_PATH={_DB_PATH_IN_CONTAINER} python3 -m bot_bottle.gateway_init ) &\n"
         "while : ; do wait ; done\n"
     )