* [TEST] Remove io_context lifecycle CI skips (Issue #400)

The intentional leak pattern applied in messaging_client now prevents
heap corruption during static destruction. This allows the previously
skipped tests to run safely in CI environments.

Changes:
- Remove TODO comments referencing Issues #315 and #348
- Remove GTEST_SKIP() for io_context lifecycle issues in CI
- Update comments to reference Issue #400 and the applied fix
- Tests now pass with CI=true environment variable

Affected tests:
- ErrorHandlingTest.ConnectToInvalidHost
- ErrorHandlingTest.ConnectToInvalidPort
- ErrorHandlingTest.ConnectionRefused
- ErrorHandlingTest.RecoveryAfterConnectionFailure
- ConnectionLifecycleTest.ClientConnectionToNonExistentServer
- MultiConnectionLifecycleTest.SequentialConnections

* docs: update CHANGELOG for io_context lifecycle fix (#400)

Document the re-enabled integration tests in both English and Korean
CHANGELOG files under the Fixed section.

* fix(test): wait for async connection attempts to complete before cleanup

Add wait_for_connection_attempt helper to test_helpers.h to ensure
async connection operations (resolve/connect) complete before test
cleanup. This prevents heap corruption that occurs when:

1. Test initiates async connection to non-existent server
2. Test exits immediately without waiting
3. TearDown calls stop_client() while async operations are in progress
4. Pending handlers access invalidated memory during cleanup

Modified tests:
- ErrorHandlingTest.ConnectToInvalidHost
- ErrorHandlingTest.ConnectToInvalidPort
- ErrorHandlingTest.ConnectionRefused
- ErrorHandlingTest.RecoveryAfterConnectionFailure
- ConnectionLifecycleTest.ClientConnectionToNonExistentServer
- MultiConnectionLifecycleTest.SequentialConnections

The fix uses error_callback to detect connection failures and waits
for either successful connection or error callback before allowing
test cleanup to proceed.

Fixes heap corruption in CI (Issue #400)

* docs: update CHANGELOG with wait_for_connection_attempt helper

Document the test helper function and test modifications that prevent
heap corruption from incomplete async connection operations.