Define AllGather.__enter/exit__ for insert_finished

[mroeschke]

In cudf_polars we often have to use a finally: AllGather.insert_finished() pattern to ensure that method is called after AllGather.insert e.g. rapidsai/cudf#22516.

This PR defines .__enter/exit__ like we do on Context to simplify this convention when we need to call AllGather.insert

[Matt711]

I think we have to be careful in the streaming case because of

rapidsmpf/cpp/include/rapidsmpf/streaming/coll/allgather.hpp

Lines 23 to 30 in d8e53ff

	/**
	* @brief Asynchronous (coroutine) interface to `coll::AllGather`.
	*
	* Once the AllGather is created, many tasks may insert data into it. If multiple tasks
	* insert data, the user is responsible for arranging that `insert_finished` is only
	* called after all `insert`ions have completed. A single consumer task should extract
	* data.
	*/

It's the callers responsibility to do this. We probably should only do the "bulk" case in this PR. And keep using AllGatherManager.Inserter in cudf-polars.

[mroeschke]

Sure we can still use the AllGatherManager.Inserter API in cudf_polars, but this would help simplify that implementation too, with this inserting could become

@contextmanager
def inserting(self):
    with self.allgather:
        yield self

and could move Inserter.insert onto AllGatherManager.insert if desired

[Matt711]

Oh ok I think that work, thanks.

[Matt711]

Yeah I think we can remove the Inserter class but we have to keep inserting cm, so the caller can control when insert_finished is called?

[mroeschke]

Yeah it's kinda nice to keep an API like inserting to signal that "after exiting the cm this calls insert_finished"