fix(agents): restore abc.ABC base for BaseAgent and LlmAgent

BaseAgent and LlmAgent re-inherit abc.ABC. v2 GA re-rooted them on
BaseNode (plain BaseModel), so static checkers no longer see an
ABCMeta base and flag @abc.abstractmethod on consumer subclasses as
stray, cascading into bad-return-type. Runtime is unchanged because
pydantic's ModelMetaclass already extends ABCMeta; this only makes
the metaclass visible to type checkers.

Change-Id: I2632a180b0616a244cba97a161c1845101acd998

Author: GWeale

src/google/adk/agents/base_agent.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import abc
 import inspect
 import logging
 from typing import Any
@@ -87,7 +88,9 @@ class BaseAgentState(BaseModel):
 AgentState = TypeVar('AgentState', bound=BaseAgentState)
 
 
-class BaseAgent(BaseNode):
+# TODO: drop the explicit abc.ABC base once BaseNode surfaces ABCMeta to
+# static type checkers.
+class BaseAgent(BaseNode, abc.ABC):
   """Base class for all agents in Agent Development Kit."""
 
   model_config = ConfigDict(

src/google/adk/agents/llm_agent.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import abc
 import asyncio
 import importlib
 import inspect
@@ -192,7 +193,9 @@ async def _convert_tool_union_to_tools(
     return []
 
 
-class LlmAgent(BaseAgent):
+# TODO: drop the explicit abc.ABC base once BaseNode surfaces ABCMeta to
+# static type checkers.
+class LlmAgent(BaseAgent, abc.ABC):
   """LLM-based Agent."""
 
   DEFAULT_MODEL: ClassVar[str] = 'gemini-3-flash-preview'