feat: allow AND for summarization trigger conditions

[sydney-runkle]

The trigger parameter now supports AND/OR logic via nested lists.
Top-level items are OR'd together, nested lists are AND'd together.

Before (OR logic only):

SummarizationMiddleware(
    ...,
    trigger=[("tokens", 3000), ("messages", 100)]
)
# Triggers when tokens >= 3000 OR messages >= 100

After (AND/OR logic):

SummarizationMiddleware(
    ...,
    trigger=[("messages", 10), [("tokens", 500), ("fraction", 0.8)]]
)
# Triggers when messages >= 10 OR (tokens >= 500 AND fraction >= 0.8)