#247 Per-activity circuit breaker configuration via DSL

Allow users to optionally configure circuit breaker per activity in DSL. Default: disabled (workflow retry_policy is source of truth). When enabled, user specifies: failure_threshold, success_threshold, isolation_duration. Example: circuit_breaker=CircuitBreakerPolicy(failure_threshold=5, isolation_duration=timedelta(seconds=30)). Requires: 1) DSL extension for circuit_breaker param, 2) Pass config to tool execution, 3) Tests for enabled/disabled scenarios.