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/command/cmd_funcs.c

@@ -11115,7 +11115,7 @@ cmd_ai_switch(ProfWin* window, const char* const command, gchar** args)
         aiwin->session->provider_name = g_strdup(provider_name);
         ai_provider_unref(aiwin->session->provider);
         aiwin->session->provider = new_provider;
-        aiwin->session->provider->ref_count++;
+        ai_provider_ref(aiwin->session->provider);
     }
     g_free(aiwin->session->model);
     aiwin->session->model = g_strdup(model);