gptel-curl: handle large Curl payloads with a temp file (#137)

gptel-curl.el (gptel-curl--get-args,
gptel-curl-file-size-threshold): Use temporary file for curl data.
Ensure curl uses a temporary file for binary data to prevent
issues with large payloads and special characters:

- Add a new defcustom `gptel-curl-file-size-threshold` to
determine when to use a temporary file for passing data to Curl.

- Use `--data-binary` with a temp file for data larger than the
specified threshold, improving handling of large data payloads in
GPTel queries.

- Reliably clean up temporary files created for Curl requests
exceeding the size threshold.  Add a function to
`gptel-post-response-hook` to delete the file post-Curl execution
and remove itself from the hook, preventing temporary file
accumulation.

gptel-curl.el

@@ -61,8 +61,18 @@ PROMPTS is the data to send, TOKEN is a unique identifier."
                       backend-header)))))
     (append
      gptel-curl--common-args
-     (list (format "-w(%s . %%{size_header})" token)
-           (format "-d%s" data))
+     (list (format "-w(%s . %%{size_header})" token))
+     (if (length< data gptel-curl-file-size-threshold)
+         (list (format "-d%s" data))
+       (letrec
+           ((temp-filename (make-temp-file "gptel-curl-data" nil ".json" data))
+            (cleanup-fn (lambda ()
+                          (when (file-exists-p temp-filename)
+                            (delete-file temp-filename)
+                            (remove-hook 'gptel-post-response-hook cleanup-fn)))))
+         (add-hook 'gptel-post-response-hook cleanup-fn)
+         (list "--data-binary"
+               (format "@%s" temp-filename))))
      (when (not (string-empty-p gptel-proxy))
        (list "--proxy" gptel-proxy
              "--proxy-negotiate"

gptel.el

@@ -163,6 +163,25 @@ all at once. This wait is asynchronous.
   :group 'gptel
   :type 'boolean)
 
+(defcustom gptel-curl-file-size-threshold 130000
+  "Size threshold for using file input with Curl.
+
+Specifies the size threshold for when to use a temporary file to pass data to
+Curl in GPTel queries. If the size of the data to be sent exceeds this
+threshold, the data is written to a temporary file and passed to Curl using the
+`--data-binary' option with a file reference. Otherwise, the data is passed
+directly as a command-line argument.
+
+The value is an integer representing the number of bytes.
+
+Adjusting this value may be necessary depending on the environment
+and the typical size of the data being sent in GPTel queries.
+A larger value may improve performance by avoiding the overhead of creating
+temporary files for small data payloads, while a smaller value may be needed
+if the command-line argument size is limited by the operating system."
+  :group 'gptel
+  :type 'integer)
+
 (defcustom gptel-response-filter-functions
   '(gptel--convert-org)
   "Abnormal hook for transforming the response from ChatGPT.