Fix serialization inconsistency in MultiStepAgent.to_dict/from_dict for managed_agents

[hazrmard]

Fixes #1576

The MultiStepAgent.to_dict() method returns managed_agents as a dict instead of a list.

The MultiStepAgent.from_dict() class method parses the managed_agents dict and turns it back into a list of MultiStepAgent instances to be passed as arguments to instantiate the class.

[albertvillanova]

Thanks for spotting the issue and proposing a fix.

Finally, I have aligned both the to_dict and from_dict methods to use a list instead of a dict for serializing managed_agents. This reduces redundant information (since the agent name is already stored within each agent's dict) and also aligns the behavior with how tools are serialized.

Final PR Changes:

• Modify from_dict to properly handle the list format and reconstruct managed agents correctly.
• Add test_multiagent_to_dict_from_dict_roundtrip to verify that round-trip serialization and deserialization work as expected.