Release prep for 3.9.0

[smahima27]

Release 3.9.0

Bumps version 3.8.1 → 3.9.0 to release the circuit breaker and adaptive timeout changes that were merged to main via PR #691 but not yet tagged/released.

Changes included in this release

• (P4DEVOPS-9438) Add circuit breaker and adaptive timeout for provider resilience (Implement circuit breaker pattern to prevent cascading failures #691)
  • New Vmpooler::CircuitBreaker class — prevents cascading failures when a provider becomes unresponsive (CLOSED → OPEN → HALF_OPEN states)
  • New Vmpooler::AdaptiveTimeout class — dynamically adjusts operation timeouts based on recent performance
  • base.rb — initializes circuit breaker and adaptive timeout per provider
  • pool_manager.rb — propagates circuit breaker state and metrics
  • promstats.rb — registers circuit_breaker.* and related Prometheus metrics
  • vmpooler.yaml.example — documents new circuit_breaker_enabled, circuit_breaker_failure_threshold, circuit_breaker_timeout, adaptive_timeout_enabled config options

Files changed

• lib/vmpooler/version.rb — 3.8.1 → 3.9.0
• CHANGELOG.md — added 3.9.0 entry
• Gemfile.lock — updated gem version reference