fix(ai): bound response parsing and harden AUTO fallback errors

- bound _parse_responses scans to the output_text part's object and to
  the content array, so a later sibling item's "text" (e.g. a reasoning
  summary) can never be returned as the assistant reply
- do not retry the other flavour on a curl timeout: the request likely
  reached the server and may still be generating, so a re-POST of the
  conversation could trigger a second billed generation
- remember an unparseable 2xx from the first AUTO attempt and surface
  stashed first-attempt errors in both error paths, instead of showing
  only the final attempt's transport or HTTP error
- recognize Ollama's model-not-found wording in _names_model so a model
  typo is not misread as a missing endpoint
- match model errors case-insensitively and recognize the
  'model "x" is not supported' wording, so a wrong-model 400 (seen live
  on Perplexity /v1/responses) fails fast instead of probing the other
  flavour with a doomed request
- say "empty response body" instead of "Unknown error" when an HTTP
  error arrives without a body (e.g. Perplexity's bare 404)
- drop the dead, racy provider-lookup fallback in the generic request
  thread: a missing provider ref is a caller bug and now fails loudly
- fix ai_providers_lists_defaults to expect the header the command
  actually prints ("Configured providers:"); the test was broken since
  its introduction but CI never ran it
- add functional test group 5 (AI command surface) to FUNC_TEST_GROUPS
  so the CI parallel target runs it; proftest.c port ranges already
  account for five groups

src/ai/ai_client.c

@@ -472,12 +472,16 @@ _names_model(const gchar* s)
 {
     if (!s)
         return FALSE;
-    /* "does not exist"/"not found" alone also appear in route-missing bodies;
+    /* "does not exist"/"not found"/"not supported" alone also appear in
-     * require them to talk about a model (covers Ollama's 'model "x" not
+     * route-missing bodies; require them to talk about a model (covers
-     * found, try pulling it first') */
+     * Ollama's 'model "x" not found, try pulling it first' and Perplexity's
-    return strstr(s, "model_not_found") || strstr(s, "invalid_model")
+     * 'model "x" is not supported'). Gateways vary in casing, so compare
-           || strstr(s, "unknown_model")
+     * case-insensitively. */
-           || (strstr(s, "model") && (strstr(s, "does not exist") || strstr(s, "not found")));
+    auto_gchar gchar* lower = g_ascii_strdown(s, -1);
+    return strstr(lower, "model_not_found") || strstr(lower, "invalid_model")
+           || strstr(lower, "unknown_model")
+           || (strstr(lower, "model")
+               && (strstr(lower, "does not exist") || strstr(lower, "not found") || strstr(lower, "not supported")));
 }
 
 /* A missing endpoint answers 404/405/501. A wrong model can also 404/405, but
@@ -1088,7 +1092,7 @@ _curl_exec_and_handle(CURL* curl, const gchar* url, struct curl_slist* headers,
         auto_gchar gchar* error_msg = parsed
                                           ? g_strdup_printf("HTTP %ld: %s", http_code, parsed)
                                           : g_strdup_printf("HTTP %ld: %s", http_code,
-                                                            response->data && strlen(response->data) > 0 ? response->data : "Unknown error");
+                                                            response->data && strlen(response->data) > 0 ? response->data : "empty response body");
         log_error("Request HTTP error for '%s': %s", provider->name, error_msg);
         _aiwin_display_error(user_data, error_msg);
     } else {
@@ -2236,7 +2240,7 @@ _ai_request_thread(gpointer data)
                   response.size, response_data);
         /* Try to extract the actual error message from the JSON response */
         auto_gchar gchar* parsed_error = ai_parse_error_message(err_body);
-        auto_gchar gchar* error_msg = parsed_error ? g_strdup_printf("HTTP %ld: %s", err_code, parsed_error) : g_strdup_printf("HTTP %ld: %s", err_code, err_body && strlen(err_body) > 0 ? err_body : "Unknown error");
+        auto_gchar gchar* error_msg = parsed_error ? g_strdup_printf("HTTP %ld: %s", err_code, parsed_error) : g_strdup_printf("HTTP %ld: %s", err_code, err_body && strlen(err_body) > 0 ? err_body : "empty response body");
         if (first_unparsable_note) {
             gchar* with_note = g_strdup_printf("%s (first attempt: %s)", error_msg, first_unparsable_note);
             g_free(error_msg);