feat(ai): support both responses and chat completions APIs

Drive both OpenAI-compatible flavours from one code path. Requests
default to /v1/responses and fall back once to /v1/chat/completions when
the provider reports the endpoint missing (404/405/501) or rejects the
payload shape (400/422); the working flavour is cached for the rest of
the run as a first-attempt hint, with the other flavour kept as a
fallback so a backend change self-corrects in-request. A per-provider
override is available via
  /ai set api-type <provider> responses|chat-completions|auto
and persisted as api_type= in the provider section.

One payload builder serves both flavours (messages/stream vs
input/stream/store); history and custom settings are serialized once per
request and the per-flavour envelope is assembled per attempt under a
single lock acquisition, so the fallback retry cannot double-count the
prompt. ai_parse_response_typed() dispatches on the request flavour and
splits extraction per envelope: chat completions anchored on
choices[].message.content, responses on the output_text part with a
string-aware backward scan bounded to that part's own object so a
reasoning summary or a truncated body is never returned as the reply.

Endpoint detection classifies wrong-model errors from the structured
error code/type/message when present, so route-missing 404s from
gateways no longer suppress the fallback; it preserves and surfaces the
first payload-rejection error when the fallback also fails, and re-probes
on an unparseable 2xx. resolved_api_type is written only through a locked
helper guarded by a URL/api-type epoch, and api_url is snapshotted under
settings_lock in the request and models-fetch threads, closing
use-after-free and stale-cache races against /ai set provider. The
models-fetch provider ref is taken on the main thread and released on
every worker exit path. Reserved custom-setting keys are extended with
input (store stays writable as a legitimate chat-completions parameter).

tests/unittests/unittests.c

@@ -753,6 +753,8 @@ main(int argc, char* argv[])
         cmocka_unit_test_setup_teardown(test_ai_get_provider_default_model, ai_client_setup, ai_client_teardown),
         cmocka_unit_test_setup_teardown(test_ai_set_provider_setting, ai_client_setup, ai_client_teardown),
         cmocka_unit_test_setup_teardown(test_ai_set_provider_setting_reserved_key, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_set_provider_api_type, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test(test_ai_api_type_string_round_trip),
         cmocka_unit_test_setup_teardown(test_ai_get_provider_setting, ai_client_setup, ai_client_teardown),
         /* Model caching tests */
         cmocka_unit_test_setup_teardown(test_ai_models_are_fresh_initial, ai_client_setup, ai_client_teardown),
@@ -775,8 +777,17 @@ main(int argc, char* argv[])
         cmocka_unit_test(test_ai_json_escape_backslash_quote),
         /* Chat response parser */
         cmocka_unit_test(test_ai_parse_response_openai_content),
-        cmocka_unit_test(test_ai_parse_response_legacy_text_format_unsupported),
+        cmocka_unit_test(test_ai_parse_response_responses_output_text),
+        cmocka_unit_test(test_ai_parse_response_responses_full_envelope),
+        cmocka_unit_test(test_ai_parse_response_responses_reasoning_summary_skipped),
+        cmocka_unit_test(test_ai_parse_response_choices_win_over_output),
+        cmocka_unit_test(test_ai_parse_response_choices_in_string_value),
+        cmocka_unit_test(test_ai_parse_response_content_before_choices),
         cmocka_unit_test(test_ai_parse_response_content_preferred_over_text),
+        cmocka_unit_test(test_ai_parse_response_responses_brace_in_text),
+        cmocka_unit_test(test_ai_parse_response_choices_logprobs_before_content),
+        cmocka_unit_test(test_ai_parse_response_reasoning_not_leaked_without_text),
+        cmocka_unit_test(test_ai_parse_response_responses_text_value_text),
         cmocka_unit_test(test_ai_parse_response_escaped_quote),
         cmocka_unit_test(test_ai_parse_response_newline_escape),
         cmocka_unit_test(test_ai_parse_response_empty_content),
@@ -831,6 +842,7 @@ main(int argc, char* argv[])
         /* Prefs round-trip (uses prefs+ai setup) */
         cmocka_unit_test_setup_teardown(test_ai_prefs_round_trip_api_key, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
         cmocka_unit_test_setup_teardown(test_ai_prefs_round_trip_remove_key, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
+        cmocka_unit_test_setup_teardown(test_ai_prefs_round_trip_api_type, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
         cmocka_unit_test_setup_teardown(test_ai_prefs_multiple_providers_persist, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
         /* Autocomplete deeper coverage */
         cmocka_unit_test_setup_teardown(test_ai_providers_find_after_remove_skips_removed, ai_client_setup, ai_client_teardown),