Fixes#1372 — phantom busy: canvas showed workspace as active for up
to 30s after task completion because set_current_task("") returned
early without posting the updated heartbeat.
Before: clearing only updated the heartbeat object; the next 30s
scheduled heartbeat cycle propagated the clear. Quick tasks would leave
a phantom-busy indicator.
After: both SET and CLEAR push immediately to /registry/heartbeat.
active_tasks=0 on clear, active_tasks=1 on set. Heartbeat object
update and HTTP post are now unconditional.
Tests: 5 new cases covering SET/CLEAR HTTP body, error resilience,
None heartbeat, and missing env vars.
Co-authored-by: Molecule AI Infra-Runtime-BE <infra-runtime-be@agents.moleculesai.app>
