Sanitize gpt-model value just in time

* gptel-transient.el (gptel-menu): Sanitize model if it's not in
the backend.

* gptel.el (gptel--sanitize-model): New helper.

* gptel.el (gptel-send): Also sanitize model in non-prefixed gptel-send.

gptel-transient.el

@@ -156,8 +156,10 @@ which see."
      (lambda () (if (derived-mode-p 'prog-mode)
                "Refactor" "Rewrite"))
      gptel-rewrite-menu)]
-   ["Send" (gptel--suffix-send)]])
-
+   ["Send" (gptel--suffix-send)]]
+  (interactive)
+  (gptel--sanitize-model)
+  (transient-setup 'gptel-menu))
 
 ;; ** Prefix for setting the system prompt.
 (defun gptel-system-prompt--setup (_)

gptel.el

@@ -859,6 +859,7 @@ waiting for the response."
   (if (and arg (require 'gptel-transient nil t))
       (call-interactively #'gptel-menu)
   (message "Querying %s..." (gptel-backend-name gptel-backend))
+  (gptel--sanitize-model)
   (let* ((response-pt
           (if (use-region-p)
               (set-marker (make-marker) (region-end))
@@ -1115,6 +1116,22 @@ See `gptel-curl--get-response' for its contents.")
         (list nil (concat "(" http-msg ") Could not parse HTTP response.")
               "Could not parse HTTP response.")))))
 
+(cl-defun gptel--sanitize-model (&key (backend gptel-backend)
+                                      (shoosh t))
+  "Check if `gptel-model' is available in BACKEND, adjust accordingly.
+
+If SHOOSH is true, don't issue a warning."
+  (let* ((available (gptel-backend-models backend)))
+    (unless (member gptel-model available)
+      (let ((fallback (car available)))
+        (unless shoosh
+          (display-warning
+           'gptel
+           (format (concat "Preferred `gptel-model' \"%s\" not"
+                           "supported in \"%s\", using \"%s\" instead")
+                   gptel-model (gptel-backend-name backend) fallback)))
+        (setq-local gptel-model fallback)))))
+
 ;;;###autoload
 (defun gptel (name &optional _ initial interactivep)
   "Switch to or start ChatGPT session with NAME.
@@ -1154,6 +1171,8 @@ INTERACTIVEP is t when gptel is called interactively."
       (text-mode)
       (visual-line-mode 1))
      (t (funcall gptel-default-mode)))
+    (gptel--sanitize-model :backend (default-value 'gptel-backend)
+                           :shoosh nil)
     (unless gptel-mode (gptel-mode 1))
     (goto-char (point-max))
     (skip-chars-backward "\t\r\n")