refactor: address code review feedback

- Remove unused TypeVars (TInput, TCallback)
- Simplify CallbackExecutor by reusing CallbackPipeline
- Reduces code duplication and improves maintainability

Addresses feedback from gemini-code-assist bot review

Author: jaywang172

src/google/adk/agents/callback_pipeline.py

@@ -42,12 +42,10 @@
 from typing import Union
 
 
-TInput = TypeVar('TInput')
 TOutput = TypeVar('TOutput')
-TCallback = TypeVar('TCallback', bound=Callable)
 
 
-class CallbackPipeline(Generic[TInput, TOutput]):
+class CallbackPipeline(Generic[TOutput]):
   """Unified callback execution pipeline.
 
   This class provides a consistent way to execute callbacks with the following
@@ -241,17 +239,10 @@ async def execute_with_plugins(
       ... )
     """
     # Step 1: Execute plugin callback (priority)
-    result = plugin_callback(*args, **kwargs)
-    if inspect.isawaitable(result):
-      result = await result
-
+    result = await CallbackPipeline([plugin_callback]).execute(*args, **kwargs)
     if result is not None:
       return result
 
     # Step 2: Execute agent callbacks if plugin returned None
-    if agent_callbacks:
-      pipeline = CallbackPipeline(callbacks=agent_callbacks)
-      result = await pipeline.execute(*args, **kwargs)
-
-    return result
+    return await CallbackPipeline(agent_callbacks).execute(*args, **kwargs)