fix(ci,infra): repair post-merge publish + harden candidate handling

Review follow-ups for the infra candidate-artifact flow:

1. publish-infra would fail on the first merge to main. The rootfs version now
   hashes the dropbear bytes, and build-infra sets BOT_BOTTLE_FC_DROPBEAR to the
   staged dropbear — but publish-infra (ubuntu-latest) set none, so _publish_
   bundle re-derived the version with a "<missing>" dropbear and rejected the
   candidate as "does not match checkout". Download the same firecracker-inputs
   dropbear and export BOT_BOTTLE_FC_DROPBEAR in publish-infra (and add
   stage-firecracker-inputs to its needs, since it now consumes that artifact).

2. Guard stage-firecracker-inputs with the same fork-PR check as the other
   KVM-runner jobs, so a fork PR can't spin the privileged runner (it only
   copies a static binary, but keep the posture consistent; it gates the whole
   Firecracker chain via needs).

3. ensure_artifact_gz / _publish_bundle read version.txt before checking it
   exists — a bundle missing it raised a raw FileNotFoundError instead of the
   intended "bundle is incomplete" die. Guard the read (kept before the gz/sha
   completeness check so a wrong-version bundle still reports the version
   mismatch, not "incomplete").

4. test_infra_artifact.setUp didn't isolate BOT_BOTTLE_INFRA_ARTIFACT_DIR, so
   an ambient candidate dir (the coverage job exports one) would send the
   registry-pull tests down the local-bundle path. Pin it off in setUp; the
   candidate-path cases set it explicitly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9qa3xoavjQScufDfZaXKR

bot_bottle/backend/firecracker/infra_artifact.py

@@ -169,7 +169,12 @@ def ensure_artifact_gz(version: str) -> Path:
     candidate_dir = os.environ.get(_CANDIDATE_DIR_ENV, "").strip()
     if candidate_dir:
         root = Path(candidate_dir)
-        declared = (root / "version.txt").read_text(encoding="utf-8").strip()
+        version_file = root / "version.txt"
+        # Guard the read so a missing version.txt is a clean error, not a raw
+        # FileNotFoundError.
+        if not version_file.is_file():
+            die(f"infra candidate bundle is incomplete: {root}")
+        declared = version_file.read_text(encoding="utf-8").strip()
         if declared != version:
             die(
                 f"infra candidate version mismatch: expected {version}, "