Prevent RuntimeError while reinitializing clusters - sync and async (#3633)

* Prevent RuntimeError while reinitializing clusters - sync and async

* Applying copilot's review comments

Author: petyaslavova

redis/asyncio/cluster.py

@@ -1315,7 +1315,9 @@ async def initialize(self) -> None:
         startup_nodes_reachable = False
         fully_covered = False
         exception = None
-        for startup_node in self.startup_nodes.values():
+        # Convert to tuple to prevent RuntimeError if self.startup_nodes
+        # is modified during iteration
+        for startup_node in tuple(self.startup_nodes.values()):
             try:
                 # Make sure cluster mode is enabled on this node
                 try:

redis/cluster.py

@@ -1674,7 +1674,9 @@ def initialize(self):
         fully_covered = False
         kwargs = self.connection_kwargs
         exception = None
-        for startup_node in self.startup_nodes.values():
+        # Convert to tuple to prevent RuntimeError if self.startup_nodes
+        # is modified during iteration
+        for startup_node in tuple(self.startup_nodes.values()):
             try:
                 if startup_node.redis_connection:
                     r = startup_node.redis_connection