Copy on co-located files (e.g., format result) does not honor max_adaptive_executor_pool_size and shared_pool_size

Once we fix #4673, Copy on co-located files (e.g., format result) does not honor max_adaptive_executor_pool_size and shared_pool_size.

We have to improve it. Some earlier ideas:
- we could consider introducing a new style of COPY that appends
- or explicitly generate multiple files and read from multiple files

Start by reviewing issue #4673 and the COPY path for co-located files such as format results. Determine how max_adaptive_executor_pool_size and shared_pool_size are currently bypassed, then settle whether completion means append-style COPY or explicitly generated multiple files that can be read together.

Written by the indexing model from the issue text.