Circuit Breaker for Stellar Client

[mftee]

Implement a circuit breaker pattern for the StellarClient to prevent cascading failures when the Stellar Horizon API is unavailable. The circuit breaker should automatically open after repeated failures and recover after a timeout.

Requirements:

• Implement a CircuitBreaker struct with states: Closed, Open, HalfOpen
• Configuration via CircuitBreakerConfig:

  pub struct CircuitBreakerConfig {
      pub failure_threshold: u32,   // failures before opening
      pub success_threshold: u32,   // successes in HalfOpen before closing
      pub timeout_secs: u64,        // seconds before trying HalfOpen
  }

• Wrap StellarClient calls: if the breaker is Open, return an error immediately without making an HTTP request
• After timeout_secs, transition to HalfOpen and allow one request through
• If that request succeeds success_threshold times, close the circuit; if it fails, reopen it
• Expose circuit state in the GET /health response
• Use std::sync::atomic types for thread-safe state management

Acceptance Criteria:

• Circuit opens after failure_threshold consecutive failures
• While open, requests fail immediately with a specific CircuitOpenError
• Unit tests for all three state transitions
• Health endpoint accurately reflects circuit state