Don't deplete all the startup nodes after ConnectionError/TimeoutError

[eoghanmurray]

Don't deplete all the startup nodes after ConnectionError/TimeoutError against all nodes, rather keep one around so that retry algorithm has at least one node to work with

Description of change

See bug report #3693

[Copilot]

Pull Request Overview

This PR updates the cluster command execution logic to avoid removing the last remaining startup node on connection or timeout failures, ensuring retries can still proceed.

• Preserve one startup node when all others fail and wrap the original exception in a RedisClusterException
• Only remove failed nodes if more than one startup node remains
• Re-raise the appropriate exception after forcing a cluster layout reinitialization

Comments suppressed due to low confidence (2)

redis/asyncio/cluster.py:824

• [nitpick] The error message could be more descriptive and grammatically clear, e.g., 'Unable to connect to Redis Cluster: connection or timeout errors on all startup nodes'.

                        'Connection or Timeout Errors across all startup nodes'

redis/asyncio/cluster.py:820

• Add a unit test covering the scenario where only one startup node remains to ensure it isn't removed and the correct RedisClusterException is raised with the original cause.

                if len(self.nodes_manager.startup_nodes) == 1:

[Copilot]

[nitpick] Reassigning the caught exception variable e to a new exception can be confusing; consider raising the new RedisClusterException directly or using a separate variable name for clarity.

Suggested change

	ce = RedisClusterException(
	'Redis Cluster cannot be connected. '
	'Connection or Timeout Errors across all startup nodes'
	)
	ce.__cause__ = e
	e = ce
	raise RedisClusterException(
	'Redis Cluster cannot be connected. '
	'Connection or Timeout Errors across all startup nodes'
	) from e