[rllib] Update flatten_observations.py for nested spaces for ignored multi-agent

[pseudo-rnd-thoughts]

Description

For the FlattenObservation connector, in the mutli-agent case, if any of the agent's had a nested space, if it was ignored then we used the base_struct version of the observation_spaces rather than the actual spaces which for nested spaces is a problem.
FYI, I believe that get_base_struct_from_space can be removed as Gymnasium has added the ability to treat Dict and Tuple spaces as like a dict or tuple for iterating, calling .keys(), .values() and .items(). For a future PR possibly

Related issues

Fixes #59849

[gemini-code-assist]

Code Review

This pull request addresses a bug in the FlattenObservations connector for multi-agent scenarios with nested observation spaces. The core fix correctly preserves the original observation space for agents whose observations are not flattened, which is crucial for nested spaces. The changes are well-implemented and include several improvements: a new multi-agent example in the docstring that clearly demonstrates the fix, added type hints for better readability, and new assertions that enhance the robustness of the code by validating space types. The docstring for the __init__ method has also been updated for better clarity. Overall, this is a high-quality contribution that not only fixes the bug but also improves the code's maintainability and documentation.