fix: Resolve circular import in base_tool

Importing `ToolContext` at the top level in `base_tool.py` created a circular import cycle when `google.adk.tools.function_tool` was imported.
The cycle was: `base_tool` -> `tool_context` -> `CallbackContext` -> `agents` -> `BaseAgent` -> `Event` -> `LlmResponse` -> `LlmRequest` -> `BaseTool`.

Moving `ToolContext` to `TYPE_CHECKING` breaks this runtime cycle, using `from __future__ import annotations` to preserve type hints.

Change-Id: I34731f46cf5aa4665ad3c6604495ad5fd84f8aa5