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/command/cmd_defs.c

@@ -2815,6 +2815,7 @@ static const struct cmd_t command_defs[] = {
               "/ai set provider <name> <url>",
               "/ai set token <provider> <token>",
               "/ai set default-model <provider> <model>",
+              "/ai set api-type <provider> chat-completions|responses|auto",
               "/ai set custom <provider> <setting> <value>",
               "/ai remove provider <name>",
               "/ai providers",
@@ -2830,9 +2831,10 @@ static const struct cmd_t command_defs[] = {
               "Chat history is maintained per session and not persisted locally.")
       CMD_ARGS(
               { "", "Display current AI settings and configured providers" },
-              { "set provider <name> <url>", "Add or update a provider with custom API endpoint" },
+              { "set provider <name> <url>", "Add or update a provider with a custom API base URL (endpoint paths are appended automatically)" },
               { "set token <provider> <token>", "Set API token for a provider (e.g., openai, perplexity)" },
               { "set default-model <provider> <model>", "Set default model for a provider" },
+              { "set api-type <provider> chat-completions|responses|auto", "Pin the provider API flavour; auto tries responses and falls back to chat completions when the endpoint is missing" },
               { "set custom <provider> <setting> <value>", "Set provider-level setting (e.g., temperature, max_tokens)" },
               { "remove provider <name>", "Remove a custom provider" },
               { "providers", "List configured providers with full details" },
@@ -2844,8 +2846,9 @@ static const struct cmd_t command_defs[] = {
               "/ai",
               "/ai set token openai sk-xxx",
               "/ai set token perplexity pplx-xxx",
-              "/ai set provider custom https://my-api.com/v1/chat/completions",
+              "/ai set provider custom https://my-api.com/",
               "/ai set default-model perplexity sonar",
+              "/ai set api-type perplexity responses",
               "/ai set custom perplexity temperature 0.7",
               "/ai remove provider custom",
               "/ai start",