gptel-transient: Use proper ellipsis for consistent UI

* gptel-transient.el (gptel-option-overlaid): Use a proper
ellipsis (…) instead of three ASCII dots (...) when truncating
the user instruction, provided Unicode is displayable and
`truncate-string-ellipsis' is nil.  This change allows for two
additional characters and enhances visual appeal and consistency.

Author: pabl0

gptel-transient.el

@@ -532,7 +532,7 @@ Also format its value in the Transient menu."
         (add-hook 'transient-exit-hook ov-clear-hook)))
     ;; Updating transient menu display
     (if value
-        (propertize (concat argument (truncate-string-to-width value 25 nil nil "..."))
+        (propertize (concat argument (truncate-string-to-width value 25 nil nil 'ellipsis))
                     'face 'transient-value)
       (propertize
        (concat "(" (symbol-name (oref obj display-nil)) ")")