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

Author: wukath

src/google/adk/tools/base_tool.py

@@ -33,13 +33,13 @@
 
 from ..utils.variant_utils import get_google_llm_variant
 from ..utils.variant_utils import GoogleLLMVariant
-from .tool_context import ToolContext
 
 logger = logging.getLogger("google_adk." + __name__)
 
 if TYPE_CHECKING:
   from ..models.llm_request import LlmRequest
   from .tool_configs import ToolArgsConfig
+  from .tool_context import ToolContext
 
 SelfTool = TypeVar("SelfTool", bound="BaseTool")