fix(ai): align ai_list_providers with documented contract

Remove ai_provider_ref() from ai_list_providers() so the returned
providers do not need to be unref'd by the caller. This matches the
header docstring which states "caller must not free the list or
providers". Also update all callers (cmd_ai_providers, tests) to
remove redundant ai_provider_unref() calls.

src/command/cmd_funcs.c

@@ -10929,7 +10929,6 @@ cmd_ai_providers(ProfWin* window, const char* const command, gchar** args)
         }
         cons_show("");
         g_free(key);
-        ai_provider_unref(provider);
     }
     g_list_free(providers);