Add circuit breaker for external service failures

[meshackyaro]

Description

External service outages cause cascading failures. Need circuit breaker pattern to fail fast.

Component

SDK

Difficulty

🔴 Hard

Tasks

• Implement circuit breaker for Horizon/RPC
• Configure failure thresholds
• Add half-open state for recovery testing
• Emit circuit breaker events
• Test with simulated outages

Acceptance Criteria

• Circuit breaker prevents cascade failures
• Service recovery detected automatically
• Circuit state observable

Estimated Time

3-4 days