[CI] heart beating ongoing chunk

[lihao-figma]

Add heartbeat mechanism to prevent stealing active tests

Previously, if a test's deadline estimation was wrong, other workers
could steal the test even while the original worker was still actively
running it. This could cause duplicate test runs and wasted resources.

This change adds a heartbeat mechanism:

1. Workers automatically send heartbeats every 10 seconds (configurable
   via heartbeat_interval) while running a test
2. Each heartbeat extends the test's deadline and records the heartbeat
   time in Redis
3. Before claiming a "lost" test, reserve_lost.lua checks if it was
   recently heartbeated (within heartbeat_grace_period, default 30s)
4. Tests with recent heartbeats are skipped, allowing the active worker
   to continue

Changes:

• Fix heartbeat.lua to properly extend deadline (was setting score to
  current_time instead of current_time + timeout)
• Add heartbeat tracking in Redis hash for "recent activity" checks
• Update reserve_lost.lua to respect heartbeat grace period
• Add automatic heartbeating via background thread in Worker#poll
• Add heartbeat_interval and heartbeat_grace_period config options
• Add logging when heartbeat extends deadline

[lihao-figma]

• [CI] heart beating ongoing chunk #83 👈 (View in Graphite)
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.