fix(tui): chain slash arg autocomplete after Tab completion

[barapa]

Implements chaining into argument completions immediately after slash-command name completion via Tab.

• After accepting /mod -> /model , the editor retriggers autocomplete once to show argument suggestions (if the command has an arg completer).
• Keeps Enter/submit behavior unchanged.

Tests:

• Added regression coverage for chaining into argument completions after Tab completion.
• Verified no arg menu is shown for commands without argument completers.

Ref: closes #1442

[RimuruW]

Wait for it!

[barapa]

Is this waiting for me to do anything? Or just for @badlogic