fix(firecracker): persist the gateway mitmproxy CA across rebuilds

The #450 CA-persistence fix was only implemented for the docker backend
(the host_gateway_ca_dir bind-mount). The firecracker gateway booted with
no persistent volume, so its mitmproxy CA lived only in the ephemeral
per-boot rootfs — every rebuild/restart minted a fresh CA that every
already-running bottle distrusted, failing egress TLS with "SSL
certificate verification failed".

Give the gateway VM a persistent CA volume, mirroring the orchestrator's
registry volume: boot with a small ext4 data_drive (_ensure_ca_volume)
and mount it at mitmproxy's confdir in the gateway guest init, before the
data plane starts, so mitmproxy reuses the on-disk CA instead of
regenerating it.

Closes #510

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

tests/unit/test_firecracker_infra_vm.py

@@ -61,7 +61,11 @@ class TestRoleInits(unittest.TestCase):
         init = infra_vm.role_init("gateway")
         self.assertNotIn("bb_role=", init)
         self.assertNotIn("bot_bottle.orchestrator", init)
-        self.assertNotIn("/dev/vdb", init)  # no registry volume on the data plane
+        # Persistent CA volume mounted at mitmproxy's confdir so the CA survives
+        # a gateway-VM rebuild (issue #450). Mounted BEFORE the data plane starts.
+        self.assertIn(f"mount -t ext4 /dev/vdb {infra_vm._GATEWAY_CA_MOUNT}", init)
+        self.assertLess(init.index("/dev/vdb"),
+                        init.index("bot_bottle.gateway.bootstrap"))
         self.assertIn("export PATH=", init)  # shared preamble
         self.assertIn("BOT_BOTTLE_GATEWAY_DAEMONS=egress,git-http,supervise", init)
         self.assertIn(f"cat {infra_vm._GUEST_GATEWAY_JWT_PATH}", init)  # host-minted JWT