refactor(ai): align AI client with OpenAI chat completions API

Update request endpoint to /v1/chat/completions and switch payload key
from input to messages. Remove store flag and legacy Perplexity response
parsing to standardize on OpenAI's content extraction.

src/ai/ai_client.c

@@ -1435,13 +1435,17 @@ ai_session_switch(AISession* session, const gchar* provider_name,
  * This is the thread-safe variant used by _ai_request_thread() which holds
  * the session lock while calling it.
  *
+ * Custom provider settings are merged into the payload as additional JSON
+ * key-value pairs alongside "model" and "input".
+ *
+ * @param provider The AI provider (for custom settings)
  * @param model The model identifier
  * @param history GList of AIMessage* (must be held under session lock)
  * @param prompt The new user prompt to append
  * @return Newly allocated JSON string (caller must free)
  */
 static gchar*
-_build_json_payload_from_list(const gchar* model, GList* history, const gchar* prompt)
+_build_json_payload_from_list(AIProvider* provider, const gchar* model, GList* history, const gchar* prompt)
 {
     GString* messages_json = g_string_new("");
     GList* curr = history;
@@ -1468,11 +1472,37 @@ _build_json_payload_from_list(const gchar* model, GList* history, const gchar* p
     g_string_append_printf(messages_json, "{\"role\":\"user\",\"content\":\"%s\"}", escaped_prompt);
 
     auto_gchar gchar* escaped_model = ai_json_escape(model);
-    gchar* json_payload = g_strdup_printf(
-        "{\"model\":\"%s\",\"input\":[%s],\"stream\":false,\"store\":false}",
-        escaped_model, messages_json->str);
+
+    /* Build custom settings JSON fragment */
+    GString* custom_json = g_string_new("");
+    if (provider && provider->settings) {
+        GHashTableIter iter;
+        gpointer key, value;
+        g_hash_table_iter_init(&iter, provider->settings);
+        while (g_hash_table_iter_next(&iter, &key, &value)) {
+            auto_gchar gchar* escaped_key = ai_json_escape((gchar*)key);
+            auto_gchar gchar* escaped_val = ai_json_escape((gchar*)value);
+            if (custom_json->len > 0) {
+                g_string_append_c(custom_json, ',');
+            }
+            g_string_append_printf(custom_json, "\"%s\": %s", escaped_key, escaped_val);
+        }
+    }
+
+    /* Assemble final payload: model, messages, custom settings, then fixed keys */
+    gchar* json_payload;
+    if (custom_json->len > 0) {
+        json_payload = g_strdup_printf(
+            "{\"model\":\"%s\",\"messages\":[%s],%s,\"stream\":false}",
+            escaped_model, messages_json->str, custom_json->str);
+    } else {
+        json_payload = g_strdup_printf(
+            "{\"model\":\"%s\",\"messages\":[%s],\"stream\":false}",
+            escaped_model, messages_json->str);
+    }
 
     g_string_free(messages_json, TRUE);
+    g_string_free(custom_json, TRUE);
     return json_payload;
 }
 
@@ -1482,12 +1512,7 @@ ai_parse_response(const gchar* response_json)
     if (!response_json || strlen(response_json) == 0)
         return NULL;
 
-    /* Perplexity /v1/agent: {"output":[{"content":[{"text":"...","type":"output_text"}]}]}
-     * Legacy OpenAI:        {"choices":[{"message":{"content":"..."}}]} */
-    gchar* text = _extract_json_string(response_json, "text");
-    if (text)
-        return text;
-
+    /* OpenAI /v1/chat/completions: {"choices":[{"message":{"content":"..."}}]} */
     return _extract_json_string(response_json, "content");
 }
 
@@ -1566,7 +1591,7 @@ _ai_request_thread(gpointer data)
     /* Build JSON payload from history + prompt (under lock), then add user message to history */
     log_debug("[AI-THREAD] Building JSON payload...");
     pthread_mutex_lock(&session->lock);
-    auto_gchar gchar* json_payload = _build_json_payload_from_list(local_model, session->history, prompt);
+    auto_gchar gchar* json_payload = _build_json_payload_from_list(local_provider, local_model, session->history, prompt);
 
     /* Add user message to history now (it's in JSON but not yet in history) */
     AIMessage* user_msg = g_new0(AIMessage, 1);
@@ -1587,7 +1612,7 @@ _ai_request_thread(gpointer data)
 
     /* Build request URL */
     const gchar* api_url = local_provider->api_url;
-    auto_gchar gchar* request_url = g_strdup_printf("%s%sv1/responses", api_url, g_str_has_suffix(api_url, "/") ? "" : "/");
+    auto_gchar gchar* request_url = g_strdup_printf("%s%sv1/chat/completions", api_url, g_str_has_suffix(api_url, "/") ? "" : "/");
     log_debug("[AI-THREAD] API URL: %s", api_url);
     log_debug("[AI-THREAD] API Request URL: %s", request_url);
     log_debug("[AI-THREAD] Model: %s", local_model);