[iris] Persist controller bootstrap script to GCE startup-script metadata (#5771)

GcpStandaloneWorkerHandle.bootstrap now writes the rendered script to
the instance's startup-script metadata before running it over SSH.
Without this, a future GCE-initiated reboot (host maintenance, manual
reset) re-runs the original script baked in at VM creation, pinning the
controller to the day-one image. Mirrors finelog's gcp_restart behavior.

Author: rjpower

lib/iris/src/iris/cluster/providers/gcp/handles.py

@@ -216,6 +216,14 @@ def set_metadata(self, metadata: dict[str, str]) -> None:
         except InfraError as e:
             logger.warning("Failed to set metadata on %s: %s", self._gce_vm_name, e)
 
+    def bootstrap(self, script: str) -> None:
+        # Persist the script as the instance's startup-script before executing it
+        # over SSH, so a future GCE-initiated reboot (host maintenance, manual
+        # reset) re-runs *this* script — not the original baked in at VM creation,
+        # which would pin the controller to the day-one image.
+        self.set_metadata({"startup-script": script})
+        super().bootstrap(script)
+
 
 class GcpSliceHandle:
     """Handle to a GCP TPU slice (pod).