Runner join fails: "session ID is required for session attributes"

Bug

Runner registration fails when joining a coordinator with distributedrunners lab enabled.

Error

On the coordinator (Garden) logs:

coordinator.runner-registration Failed to create node entity │ error: "remote error: generic unknown: failed to create entity in put: session ID is required for session attributes" runner_id: 0193d075-17c3-49f0-ad84-04b5b5d5d3a1

On the runner:

root@miren-garden-runner-1:~# MIREN_LABS=distributedrunners miren runner join -c 34.122.229.118:8443
Joining coordinator at 34.122.229.118:8443
ERROR: join failed: failed to register runner

Reproduction

1. Coordinator running with MIREN_LABS=adminapi,routeoidc,distributedrunners (Garden, version main:937cf5b)
2. Runner VM with miren installed (version main:937cf5b or main:d36d93a)
3. Create invite on coordinator: miren runner invite -e 24
4. On runner: miren runner join -c <coordinator>:8443 → enter join code → fails

Root cause

The runner registration code path in the coordinator is creating a node entity without providing the required session ID for session attributes. Likely the registration handler needs to either create a session first or attach to an existing one before writing the node entity to etcd.

Environment

• Coordinator: miren-garden (main:937cf5b, 2026-03-24)
• Runner: miren-garden-runner-1 (main:937cf5b)
• GCP project: miren-development