Fix %s expansion in prompts defined with define-kagi-fastgpt-prompt

The expansion is done with replace-regexp-in-string, which does a
combination of string-match and replace-match. When a function is
passed to replace-regexp-in-string instead of a text, it will call
kagi--get-text-for-prompt which uses the minibuffer / completing read,
possibly messing up the (match-data) along the way that
replace-regexp-in-string relies on. Therefore, save the matching data
before calling the supplied text function, so it will be restored and
replace-regexp-in-string won't notice it.

README.org

@@ -268,6 +268,10 @@ The full changelog can be found in README.org.
 
 - Show a welcome message when starting the FastGPT shell (=kagi-fastgpt-shell=). This can be customized with =kagi-fastgpt-welcome-function=, a variable with a function that returns a welcome string.
 
+*** Fixes
+
+- Prompts defined with =define-kagi-fastgpt-prompt= wouldn't always work when expanding =%s= placeholders (possibly resulting in =Lisp error: (args-out-of-range 0 7)=).
+
 *** Misc
 
 - Minor refactoring in the code calling the FastGPT API.

kagi.el

@@ -465,7 +465,9 @@ user input."
                               (lambda (match)
                                 (pcase match
                                   ("%%" "%")
-                                  ("%s" (or user-text (setq user-text (funcall text-function))))
+                                  ("%s" (or user-text
+                                            (setq user-text (save-match-data
+                                                              (funcall text-function)))))
                                   (_ match)))
                               prompt t t)))