Allow either threshold to be set to None to cleanly disable that trigger,
instead of requiring users to set a very large number as a workaround.
At least one of the two must remain set (validated at construction time).
The dedicated LLM logic lived in LLMAssistantAggregator, creating two
code paths and requiring the aggregator to call a private LLMService
method. Move it into the summarizer which already owns the config and
summarization lifecycle, keeping the aggregator handler as a single-line
upstream push.
