fix(ai): use atomic ai_provider_ref() in cmd_ai_switch

src/command/cmd_funcs:11118 used a plain non-atomic ref_count++ on
ai_provider, which is a data race against concurrent
ai_provider_unref() calls using g_atomic_int_dec_and_test.

- Expose ai_provider_ref() in ai_client.h (was static)
- Replace non-atomic ref_count++ with ai_provider_ref() in cmd_ai_switch"

src/ai/ai_client.c

@@ -205,7 +205,7 @@ ai_provider_new(const gchar* name, const gchar* api_url, const gchar* org_id)
     return provider;
 }
 
-static AIProvider*
+AIProvider*
 ai_provider_ref(AIProvider* provider)
 {
     if (provider) {