Ensure buffer exists before updating context

Ensured that the buffer `ellama--context-buffer` is created if it does not exist
before attempting to update and show context. This prevents errors related to
non-existent buffers. Also, added calls to `ellama-update-context-show` in both
header line and mode line minor modes to ensure context is shown when these
modes are toggled.

ellama.el

@@ -1123,7 +1123,7 @@ the context."
   "Update and show context in posframe of header line."
   (declare-function posframe-show "ext:posframe")
   (declare-function posframe-hide "ext:posframe")
-  (with-current-buffer ellama--context-buffer
+  (with-current-buffer (get-buffer-create ellama--context-buffer)
     (erase-buffer)
     (if ellama--global-context
 	(insert (format
@@ -1168,6 +1168,7 @@ the context."
 (define-minor-mode ellama-context-header-line-mode
   "Toggle Ellama Context header line mode."
   :group 'ellama
+  (ellama-update-context-show)
   (add-hook 'window-state-change-hook #'ellama-context-update-header-line)
   (if ellama-context-header-line-mode
       (ellama-context-update-header-line)
@@ -1192,6 +1193,7 @@ the context."
 (define-minor-mode ellama-context-mode-line-mode
   "Toggle Ellama Context mode line mode."
   :group 'ellama
+  (ellama-update-context-show)
   (add-hook 'window-state-change-hook #'ellama-context-update-mode-line)
   (if ellama-context-mode-line-mode
       (ellama-context-update-mode-line)