feat: process push notifications before returning connections from pool

Implement push notification processing in baseClient._getConn() to ensure
that all cluster topology changes are handled immediately before connections
are used for commands. This is critical for hitless upgrades and real-time
cluster state awareness.

Key Enhancements:

1. Enhanced Connection Retrieval (_getConn):
   - Process push notifications for both existing and new connections
   - Added processPushNotifications() call before returning connections
   - Ensures immediate handling of cluster topology changes
   - Proper error handling with connection removal on processing failures

2. Push Notification Processing Method:
   - Added processPushNotifications() method to baseClient
   - Only processes notifications for RESP3 connections with processors
   - Uses WithReader() to safely access connection reader
   - Integrates with existing push notification infrastructure

3. Connection Flow Enhancement:
   - Existing connections: Health check → Push notification processing → Return
   - New connections: Initialization → Push notification processing → Return
   - Failed processing results in connection removal and error return
   - Seamless integration with existing connection management

4. RESP3 Protocol Integration:
   - Protocol version check (only process for RESP3)
   - Push processor availability check
   - Graceful handling when processors are not available
   - Consistent behavior with existing push notification system

5. Error Handling and Recovery:
   - Remove connections if push notification processing fails
   - Return errors to trigger connection retry mechanisms
   - Maintain connection pool health and reliability
   - Prevent returning connections with unprocessed notifications

Implementation Details:
- processPushNotifications() checks protocol and processor availability
- Uses cn.WithReader() to safely access the connection reader
- Calls pushProcessor.ProcessPendingNotifications() for actual processing
- Applied to both pooled connections and newly initialized connections
- Consistent error handling across all connection retrieval paths

Flow Enhancement:
1. Connection requested via _getConn()
2. Connection retrieved from pool (existing or new)
3. Connection initialization (if new)
4. Push notification processing (NEW)
5. Connection returned to caller
6. Commands executed with up-to-date cluster state

Benefits:
- Immediate cluster topology awareness before command execution
- Enhanced hitless upgrade reliability with real-time notifications
- Reduced command failures during cluster topology changes
- Consistent push notification handling across all connection types
- Better integration with Redis cluster operations

This ensures that Redis cluster topology changes (MOVING, MIGRATING,
MIGRATED, FAILING_OVER, FAILED_OVER) are always processed before
connections are used, providing the foundation for reliable hitless
upgrades and seamless cluster operations.

Author: ndyakov

redis.go

@@ -273,6 +273,13 @@ func (c *baseClient) _getConn(ctx context.Context) (*pool.Conn, error) {
 	}
 
 	if cn.Inited {
+		// Process all pending push notifications before returning the connection
+		// This ensures that cluster topology changes are handled immediately
+		if err := c.processPushNotifications(ctx, cn); err != nil {
+			// If push notification processing fails, remove the connection
+			c.connPool.Remove(ctx, cn, err)
+			return nil, err
+		}
 		return cn, nil
 	}
 
@@ -284,9 +291,32 @@ func (c *baseClient) _getConn(ctx context.Context) (*pool.Conn, error) {
 		return nil, err
 	}
 
+	// Process any pending push notifications on the newly initialized connection
+	// This ensures that any notifications received during connection setup are handled
+	if err := c.processPushNotifications(ctx, cn); err != nil {
+		// If push notification processing fails, remove the connection
+		c.connPool.Remove(ctx, cn, err)
+		return nil, err
+	}
+
 	return cn, nil
 }
 
+// processPushNotifications processes all pending push notifications on a connection
+// This ensures that cluster topology changes are handled immediately before the connection is used
+func (c *baseClient) processPushNotifications(ctx context.Context, cn *pool.Conn) error {
+	// Only process push notifications for RESP3 connections with a processor
+	if c.opt.Protocol != 3 || c.pushProcessor == nil {
+		return nil
+	}
+
+	// Use WithReader to access the reader and process push notifications
+	// This is critical for hitless upgrades to work properly
+	return cn.WithReader(ctx, 0, func(rd *proto.Reader) error {
+		return c.pushProcessor.ProcessPendingNotifications(ctx, rd)
+	})
+}
+
 func (c *baseClient) newReAuthCredentialsListener(poolCn *pool.Conn) auth.CredentialsListener {
 	return auth.NewReAuthCredentialsListener(
 		c.reAuthConnection(poolCn),