gptel-context: add file path expansion to prevent duplicates

This prevents issues where the same file might be added multiple times
due to different path representations (e.g., relative vs absolute
paths, or paths with . and .. components).

This resolves an issue where the following gptel-context-move-to-tail
function would add the same file multiple times when the
buffer-file-name and the file path in gptel-context differed due to
path normalization.

```
(defun gptel-context-move-to-tail ()
  "Move the current buffer's file to the tail of the context."
  (when (and gptel-context
             (member (buffer-file-name) (apply #'append gptel-context)))
    (gptel-context-remove (buffer-file-name))
    (gptel-add-file (buffer-file-name))))
(add-hook 'after-save-hook #'gptel-context-move-to-tail)
(add-hook 'after-revert-hook #'gptel-context-move-to-tail)
```

Author: aagit

gptel-context.el

@@ -253,11 +253,12 @@ ACTION should be either `add' or `remove'."
 If PATH is a directory, recursively add all files in it.  PATH should be
 readable as text."
   (interactive "fChoose file to add to context: ")
-  (cond ((file-directory-p path)
-         (gptel-context--add-directory path 'add))
-	((gptel--file-binary-p path)
-         (gptel-context--add-binary-file path))
-	(t (gptel-context--add-text-file path))))
+  (let ((path (expand-file-name path)))
+    (cond ((file-directory-p path)
+           (gptel-context--add-directory path 'add))
+          ((gptel--file-binary-p path)
+           (gptel-context--add-binary-file path))
+          (t (gptel-context--add-text-file path)))))
 
 ;;;###autoload (autoload 'gptel-add-file "gptel-context" "Add files to gptel's context." t)
 (defalias 'gptel-add-file #'gptel-context-add-file)