gptel: Set gptel property as rear-nonsticky globally

* gptel.el: Set the gptel text property as rear-nonsticky
globally, by editing the default value of
`text-property-default-nonsticky'.  This variable is buffer-local,
and so gptel never did what we claimed it did, until now: text
typed at the end of a response region is not folded into the
response.

A source of bugs for many years!

Author: karthink

gptel.el

@@ -654,7 +654,8 @@ file."
 ;;; Minor mode and UI
 
 ;; NOTE: It's not clear that this is the best strategy:
-(add-to-list 'text-property-default-nonsticky '(gptel . t))
+(cl-pushnew '(gptel . t) (default-value 'text-property-default-nonsticky)
+            :test #'equal)
 
 (defun gptel--inherit-stickiness (beg end _pre)
   "Mark any change to an LLM response region as a response.