Slash command dropdown selection submits immediately — no way to add arguments for parameterized custom commands

When typing / to open the slash-command menu and selecting a custom command (e.g. a project-defined command that requires an argument like a work-item number) from the dropdown — via click, Tab, or Enter on a highlighted item — the command is submitted immediately, without giving the user a chance to type the required argument first.

This forces users to either (a) type the full command manually without ever touching the dropdown, or (b) submit without the argument and get a reprompt asking for it, wasting a round trip.

Requested behavior: distinguish "insert into input box" (so the user can then type an argument) from "insert and submit". E.g. Tab/click inserts the command text + trailing space into the input box without sending; Enter (on the input, not the dropdown) submits.

Context: VS Code extension, custom project slash command with a required parameter (e.g. /pickup-workitem <work-item-number>).