Refactor function callback handling and update type signatures

- Simplify variable names in `functions.py`: always use `function_response` and `altered_function_response`
- Update LlmAgent callback type aliases to support async:
  - Import `Awaitable`
  - Change `BeforeToolCallback` and `AfterToolCallback` signatures to return `Awaitable[Optional[dict]]`
- Ensure `after_tool_callback` uses `await` when necessary

src/google/adk/agents/llm_agent.py

@@ -15,12 +15,7 @@
 from __future__ import annotations
 
 import logging
-from typing import Any
-from typing import AsyncGenerator
-from typing import Callable
-from typing import Literal
-from typing import Optional
-from typing import Union
+from typing import Any, AsyncGenerator, Awaitable, Callable, Literal, Optional, Union
 
 from google.genai import types
 from pydantic import BaseModel
@@ -62,11 +57,11 @@ AfterModelCallback: TypeAlias = Callable[
 ]
 BeforeToolCallback: TypeAlias = Callable[
     [BaseTool, dict[str, Any], ToolContext],
-    Optional[dict],
+    Awaitable[Optional[dict]],
 ]
 AfterToolCallback: TypeAlias = Callable[
     [BaseTool, dict[str, Any], ToolContext, dict],
-    Optional[dict],
+    Awaitable[Optional[dict]],
 ]
 
 InstructionProvider: TypeAlias = Callable[[ReadonlyContext], str]

src/google/adk/flows/llm_flows/functions.py

@@ -237,12 +237,11 @@ async def handle_function_calls_live(
     #       tool, function_args, tool_context
     #   )
     if agent.before_tool_callback:
-      _maybe = agent.before_tool_callback(
-        tool=tool, args=function_args, tool_context=tool_context
-        )
-      if inspect.isawaitable(_maybe):
-        _maybe = await _maybe
-        function_response = _maybe
+      function_response = agent.before_tool_callback(
+          tool=tool, args=function_args, tool_context=tool_context
+      )
+      if inspect.isawaitable(function_response):
+        function_response = await function_response
 
     if not function_response:
       function_response = await _process_function_live_helper(
@@ -260,16 +259,16 @@ async def handle_function_calls_live(
     #   if new_response:
     #     function_response = new_response
     if agent.after_tool_callback:
-      _maybe2 = agent.after_tool_callback(
-        tool=tool,
-        args=function_args,
-        tool_context=tool_context,
-        tool_response=function_response,
-        )
-      if inspect.isawaitable(_maybe2):
-        _maybe2 = await _maybe2
-      if _maybe2 is not None:
-        function_response = _maybe2
+      altered_function_response = agent.after_tool_callback(
+          tool=tool,
+          args=function_args,
+          tool_context=tool_context,
+          tool_response=function_response,
+      )
+      if inspect.isawaitable(altered_function_response):
+        altered_function_response = await altered_function_response
+      if altered_function_response is not None:
+        function_response = altered_function_response
 
     if tool.is_long_running:
       # Allow async function to return None to not provide function response.