Implement circuit breaker pattern to prevent cascading failures

[smahima27]

• Implement circuit breaker pattern to prevent cascading failures

  • Opens after 5 consecutive failures (configurable)
  • Half-open testing after 30s timeout
  • Fail-fast to prevent thread exhaustion

• Add adaptive timeout mechanism

  • Adjusts based on p95 latency + 50% buffer
  • Reduces timeout on failures for faster fail
  • Bounded by configurable min/max values

• Add connection pool health monitoring

  • Track utilization, waiting threads, available connections
  • Emit metrics for critical/warning states
  • Monitor circuit breaker and timeout stats

• Integrate into base provider class

  • Auto-initialize if enabled in config
  • Provider-specific settings override global defaults
  • Backward compatible (enabled by default)

• Add configuration options to vmpooler.yaml.example

  • circuit_breaker_* settings
  • connection_pool_timeout_* settings
  • connection_pool_monitor_* settings

Addresses cascading failure scenario where provider outages cause
thread exhaustion and system-wide outage despite other providers
being healthy. Enables graceful degradation and auto-recovery.