Add ability to select the connection/database context for an existing .sql file.

I can create a new query by selecting New Query and then save that file, but can't open an existing .sql file and pick a connection/database under which to execute that file.

Sorry if this is possible and I'm not seeing how.

Start by comparing the existing New Query flow with the behavior when opening an existing .sql file in VS Code. Trace how a connection and database are selected, then verify that an opened file can use that context for execution; done means the selection and execution work for existing SQL files.

Written by the indexing model from the issue text.