fix(ble_log_console): resolve mypy errors for Python <3.10 compatibility

Add 'from __future__ import annotations' to defer annotation evaluation,
replace @dataclass(slots=True) with @dataclass, use typing.Union for
runtime type aliases, and suppress textual Screen metaclass false positive.

tools/bt/ble_log_console/tests/test_traffic_spike.py

@@ -1,5 +1,6 @@
 # SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
 # SPDX-License-Identifier: Apache-2.0
+from __future__ import annotations
 
 from unittest.mock import patch