omicron-stress can trigger sled-agent panic: "should have an active Propolis zone by now"

Running omicron-stress with the patch

diff --git a/src/actor/instance.rs b/src/actor/instance.rs
index 7c14b94..cd0593b 100644
--- a/src/actor/instance.rs
+++ b/src/actor/instance.rs
@@ -120,7 +120,7 @@ impl InstanceActor {
             ncpus: oxide::types::InstanceCpuCount(1),
             network_interfaces:
                 oxide::types::InstanceNetworkInterfaceAttachment::None,
-            start: true,
+            start: false,
             user_data: String::new(),
             ssh_public_keys: None,
         };

and the invocation:

exec cargo run --release -- \
    --num-test-instances 8 \
    --threads-per-instance 8 \
    --num-test-disks 0 \
    --threads-per-disk 0 \
    --num-test-snapshots 0 \
    --threads-per-snapshot 0 \
    --server-errors-fatal

In the Canada region, it seems I can reliably get sled-agent to panic after a few invocations:

thread 'tokio-rt-worker' (30) panicked at sled-agent/src/instance.rs:2286:9:
should have an active Propolis zone by now
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Reproduce the failure with the shown cargo run --release invocation and the start: false change in src/actor/instance.rs. Then inspect sled-agent/src/instance.rs around line 2286 and trace the instance lifecycle leading to the panic. Done means repeated omicron-stress runs no longer trigger the reported sled-agent panic.