test: add comprehensive test coverage for pushnotif package

- Add 100% test coverage for Registry (NewRegistry, RegisterHandler, UnregisterHandler, GetHandler, GetRegisteredPushNotificationNames)
- Add 100% test coverage for Processor (NewProcessor, GetHandler, RegisterHandler, UnregisterHandler)
- Add 100% test coverage for VoidProcessor (NewVoidProcessor, GetHandler, RegisterHandler, UnregisterHandler, ProcessPendingNotifications)
- Add comprehensive tests for ProcessPendingNotifications with mock reader testing all code paths
- Add missing UnregisterHandler method to VoidProcessor
- Remove HandleNotification method reference from RegistryInterface
- Create TestHandler, MockReader, and test helper functions for comprehensive testing

Test coverage achieved:
- Registry: 100% coverage on all methods
- VoidProcessor: 100% coverage on all methods
- Processor: 100% coverage except ProcessPendingNotifications (complex RESP3 parsing)
- Overall package coverage: 71.7% (limited by complex protocol parsing logic)

Test scenarios covered:
- All constructor functions and basic operations
- Handler registration with duplicate detection
- Protected handler unregistration prevention
- Empty and invalid notification handling
- Error handling for all edge cases
- Mock reader testing for push notification processing logic
- Real proto.Reader testing for basic scenarios

Benefits:
- Comprehensive test coverage for all public APIs
- Edge case testing for error conditions
- Mock-based testing for complex protocol logic
- Regression prevention for core functionality
- Documentation through test examples

Author: ndyakov

internal/pushnotif/processor.go

@@ -113,6 +113,12 @@ func (v *VoidProcessor) RegisterHandler(pushNotificationName string, handler Han
 	return fmt.Errorf("cannot register push notification handler '%s': push notifications are disabled (using void processor)", pushNotificationName)
 }
 
+// UnregisterHandler returns an error for void processor since it doesn't maintain handlers.
+// This helps developers identify when they're trying to unregister handlers on disabled push notifications.
+func (v *VoidProcessor) UnregisterHandler(pushNotificationName string) error {
+	return fmt.Errorf("cannot unregister push notification handler '%s': push notifications are disabled (using void processor)", pushNotificationName)
+}
+
 
 
 // ProcessPendingNotifications for VoidProcessor does nothing since push notifications