…clients (#420)

* feat(utils): implement circuit breaker pattern for fault tolerance

Add circuit_breaker class with three-state machine (closed, open, half-open)
to prevent cascade failures and provide fast-fail behavior when backend
services are unavailable.

Key features:
- Configurable failure threshold, open duration, and half-open parameters
- Thread-safe state transitions with atomic operations
- State change callbacks for monitoring/metrics integration
- Automatic transition from open to half-open after timeout

Refs: #403

* feat(utils): integrate circuit breaker with resilient_client

- Add circuit_breaker parameter to resilient_client constructor
- Check circuit breaker before attempting send operations
- Record success/failure to circuit breaker after operations
- Add circuit_state(), reset_circuit(), set_circuit_state_callback() methods
- Add circuit_open error code for fast-fail scenarios
- Move circuit_breaker_config struct outside class to fix default initializer issue

Refs: #403

* test(utils): add comprehensive circuit breaker unit tests

Add 18 unit tests covering:
- Basic state (closed, open, half-open)
- State transitions and thresholds
- Timeout-based transitions
- State change callbacks
- Reset functionality
- Thread safety with concurrent operations
- State to string conversion
- Next attempt time validation

Refs: #403

* docs(changelog): add circuit breaker implementation entry

Add changelog entries for Circuit Breaker pattern implementation (#403):
- English and Korean versions updated
- Describes circuit_breaker class with three-state pattern
- Lists all configuration parameters and API additions
- Mentions resilient_client integration and test coverage

* fix(utils): add error_codes_ext namespace to fallback build path

Add error_codes_ext namespace with circuit_open error code to the
fallback block (when KCENON_WITH_COMMON_SYSTEM is not defined) in
result_types.h for API compatibility.

This fixes build failures when building without common_system
dependency, where resilient_client.cpp references error_codes_ext
which was only defined in the KCENON_WITH_COMMON_SYSTEM path.

* docs(changelog): add circuit breaker build fix entry

Document the fix for error_codes_ext namespace missing in fallback
build path. This fix ensures builds without common_system dependency
compile correctly.