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