fix: insert-mode <Tab> inserts real tab when completion menu is not visible

[StefanBartl]

Problem:
In Insert mode, pressing with nvim-cmp mapped for completion could
produce broken behavior: the first character of the word remains in place
and extra spaces appear. This happens when the completion menu is not
visible, because the fallback() function does not insert a real tab.

Solution:

• Added mapping in the insert-mode preset (i and s modes).
• If the completion menu is visible, selects the next item as before.
• If the menu is not visible, inserts a real tab character, independent
  of expandtab, softtabstop, or other settings.
• Command-line preset ( in c mode) keeps its previous behavior, now
  consistent with insert-mode.

Impact:

• Fixes the Insert-mode bug for all users.
• Maintains full compatibility with snippet plugins and completion navigation.