fix terminal suggest bug where unexpected completions are returned

[meganrogge]

• We were adding the spec completion item even when there was no matching available command because of a startsWith check - for example,python3.13 in availableCommands would be the reason we add the python spec as a command completion. Now, we ensure there's an exact match. for python, this means it won't show up unless a user has defined that alias.
• We were checking if specLabel === command.label when command.label can be an object.
• We weren't hiding the widget when there are 0 provided completions when not manually invoked, IE typing ./, so it would stick around with the latest model's values.

fixes #241909
fixes #241964