fix(infra-artifact): include pyproject.toml in rootfs version digest

Dockerfile.gateway COPYs pyproject.toml into /src and runs pip install
/src, so it is a real input to the baked rootfs. A dependency-only change
previously reused stale artifact versions, potentially booting a rootfs
whose installed packages differed from the current checkout.

Also adds _fake_repo fixture support and a regression test so this input
can't silently drop out of the hash again.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/backend/firecracker/infra_artifact.py

@@ -85,6 +85,8 @@ def infra_artifact_version(init_script: str, *, repo_root: Path = _REPO_ROOT) ->
         h.update(name.encode())
         h.update(b"\0")
         h.update((repo_root / name).read_bytes())
+    h.update(b"pyproject.toml\0")
+    h.update((repo_root / "pyproject.toml").read_bytes())
     h.update(b"init\0")
     h.update(init_script.encode())
     return h.hexdigest()[:16]