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

tests/functionaltests/test_ai.c

@@ -55,11 +55,11 @@ ai_no_args_shows_help(void** state)
 void
 ai_providers_lists_defaults(void** state)
 {
-    /* `/ai providers` (no "list") shows the built-in list with URLs. */
+    /* `/ai providers` (no "list") shows the configured list with URLs. */
     prof_input("/ai providers");
 
     prof_timeout(5);
-    assert_true(prof_output_exact("Available AI providers:"));
+    assert_true(prof_output_exact("Configured providers:"));
     /* At least one URL line is rendered — exact name agnostic. */
     assert_true(prof_output_regex("https?://"));
     prof_timeout_reset();