registry: reduce health tracker log noise

[raphael]

Summary

Reduces logging noise from the health tracker subsystem while preserving actionable error/warning logs.

Changes

Removed high-frequency logs:

• Info logs for StartPingLoop / StopPingLoop (logged every toolset registration)
• Info log for toolset recovered (health flapping can spam this)

Added transition-based logging:

• Track observed health state per toolset
• Only emit warnings when health transitions from healthy → unhealthy
• Avoids repeated warnings for persistently unhealthy toolsets

Fixed error handling:

• StopPingLoop was using _, _ = ... to discard delete errors
• Now properly logs delete failures

Made Logger opt-in via Config:

• Config.Logger allows callers to inject a logger for health tracking
• When nil, health tracking logs are suppressed entirely

Provider error logging:

• Added error logging for pong failures in provider.Serve

What's preserved

• Warn: "toolset became unhealthy" (with full context: threshold, interval, last pong, age)
• Error: publish ping failures, delete failures, timestamp parse failures, pong failures

Testing

• make lint ✅
• make test ✅