fix(experimental): allow AgentExecutor restore from checkpoint

llm and prompt were declared required with exclude=True, making the
model un-restorable from its own serialized output. Mirror the
CrewAgentExecutor pattern: make them nullable with default None, keep
exclude=True, and re-attach llm on the resume path alongside the other
re-attached fields. Guard the two prompt-deref sites so the runtime
invariant survives the looser type.