The `get_conversation_metadata_by_name` tool retrieves conversation
metadata from another tool, `list_conversations_metadata`, but was
accessing the `next_cursor` using the Slack API response dict structure,
instead of the tool response structure. As a result, in that tool, the
tool would never actually paginate to the second page. This PR fixes it
and also adjust tests to capture the issue appropriately.
27d8aa7f43