[net-685] last applied assignment in messages

[toschoosqd]

Summary

Adds optional last_applied_assignment_id to worker heartbeat messages for NET-685.

This lets workers report the highest worker assignment they have fully applied, so the ping collector can persist that signal for later MVCC portal-promotion logic.

Notes

• Field is optional for backward compatibility.
• Field type is string; the value is opaque to workers, portals, and network-components.
• Uses proto field number 6; field 5 is already used by current_epoch.
• Assignment ordering is scheduler-owned and handled separately under NET-686.
• Scheduler-side consumption is out of scope for this PR.

Validation

• cargo test -p sqd-messages

[define-null]

LGTM