ax(mining): replace banned errors import with NewMiningError in circuit_breaker.go

ErrCircuitOpen used errors.New (banned import). Replaced with
NewMiningError using the package's own error infrastructure.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/circuit_breaker.go

@@ -1,7 +1,6 @@
 package mining
 
 import (
-	"errors"
 	"sync"
 	"time"
 
@@ -68,7 +67,8 @@ type CircuitBreaker struct {
 }
 
 // ErrCircuitOpen is returned when the circuit is open
-var ErrCircuitOpen = errors.New("circuit breaker is open")
+// if err == ErrCircuitOpen { /* fallback to cached result */ }
+var ErrCircuitOpen = NewMiningError(ErrCodeServiceUnavailable, "circuit breaker is open")
 
 // NewCircuitBreaker creates a new circuit breaker
 func NewCircuitBreaker(name string, config CircuitBreakerConfig) *CircuitBreaker {