Closes #5058

Summary

Adds event notification support to Poco::Redis::Client for connect, disconnect, and error events. This enables Node.js-style event handling for Redis client state changes.

New Components

• RedisNotification - Base notification class
• RedisConnectNotification - Fired on successful connection
• RedisDisconnectNotification - Fired on disconnect
• RedisErrorNotification - Fired on connection errors

Usage

auto pNC = client.notificationCenter();
NObserver<MyClass, RedisConnectNotification> obs(*this, &MyClass::onConnect);
pNC->addObserver(obs);
client.connect("localhost", 6379);  // Fires RedisConnectNotification

Original Implementation

Based on commits from devs/macchina.io repository:

• d1ea231 - Initial notification support with AsyncNotificationCenter member
• 18e06d3 - Lazy notification center creation with static std::once_flag
• 3f63e1a - Changed to std::atomic<std::shared_ptr<AsyncNotificationCenter>>

Differences from Original

C++20 Feature Detection

Added POCO_REDIS_HAVE_ATOMIC_SHARED_PTR macro to conditionally use std::atomic<std::shared_ptr<T>> when available, falling back to std::atomic<T*> otherwise. This ensures compatibility with compilers that don't yet support the C++20 atomic shared_ptr specialization.

Test plan

• NotificationTest::testRedisNotifications - Basic notification creation
• NotificationTest::testAsyncNotificationCenter - Async notification delivery
• NotificationTest::testClientNotifications - Client integration
• Build passes on macOS with clang (C++20 without atomic shared_ptr support)