idea: Make `list` required to return `content-length`

OpenDAL previously handled returning `content-length` in the `list` response as a best-effort approach. We encouraged users to call `stat` outside of `list`. However, this approach has placed a significant burden on our users. Perhaps we should make it mandatory for `list` to return `content-length` and update our implementations accordingly. This way, our users won't be confused by this behavior anymore.

Review the existing list response contract and how storage implementations currently provide content-length, using the issue’s contrast with stat as the starting point. Define the mandatory behavior and update affected implementations; done means list consistently returns content-length without requiring a separate stat call.

Written by the indexing model from the issue text.