fix: tui: Make /-prefixed prompt submission check if command exists

[veracioux]

Closes #3714.

[thdxr]

can you help me understand why using display isn't enough? would try to avoid introducing a new concept if possible. also don't think we need to move the autocomplete to a signal, i think we can just fetch the command dynamically

[veracioux]

@thdxr A few reasons for introducing trigger in addition to display

• For commands, display contains trailing padding, and using it for matching a command from the textarea input is problematic, We could use display.trim() but IMO it's better to restrict display to presentation only, and avoid implicit assumptions about it
• I think it's better to be explicit about what the trigger is and the kinds values it can take, including the type constraint I added to validate it as a string literal starting with @ or /
• For files, display does not contain @, whereas the trigger does (not relevant for this PR, but a nice-to-have regarding prior point about explicitness)

Point taken about the autocomplete signal, I'll make a change regarding that.

[veracioux]

Reverted changing autocomplete to a signal. Now it's a variable again.