Automatic LLM Client Resolution for Activities

[bibryam]

Description
Agents already resolve an LLM client automatically when one is not specified. They fall back to the dapr chat client, then to a conversation component. If exactly one LLM component exists, it is selected.

I want LLM activities to use the same resolution logic, so I do not need to create an LLM client explicitly.

From this

llm = DaprChatClient(component_name="openai")

@llm_activity(
    prompt="Create a very short outline about the topic '{topic}'. Provide 5 bullet points only.",
    llm=llm,
)
 

To this as default behaviour

@llm_activity(
    prompt="Create a very short outline about the topic '{topic}'. Provide 5 bullet points only."
)
 

This will align the existing LLM resolution logic between the agent and workflows