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).

src/config/preferences.h

@@ -368,6 +368,11 @@ gboolean prefs_ai_set_default_model(const char* const provider, const char* cons
 gboolean prefs_ai_remove_default_model(const char* const provider);
 char* prefs_ai_get_default_model(const char* const provider);
 
+/* AI API type per provider ("responses" or "chat-completions") */
+gboolean prefs_ai_set_api_type(const char* const provider, const char* const api_type);
+gboolean prefs_ai_remove_api_type(const char* const provider);
+char* prefs_ai_get_api_type(const char* const provider);
+
 /* AI per-provider custom settings (e.g. tools, search) — persisted as
  * setting.<name>=<value> inside [ai/<provider>]. Pass NULL value to remove. */
 gboolean prefs_ai_set_setting(const char* const provider, const char* const setting, const char* const value);