fix: reduce listener leak from per-part show-checkmarks subscriptions (#301186)

Move the show-checkmarks class toggle from individual content parts to
the row container level in chatListRenderer. This replaces N per-part
onDidChangeConfiguration subscriptions (one per tool invocation, terminal
tool, and code block pill) with a single subscription per rendered chat
element, significantly reducing listener accumulation on the configuration
service emitter in sessions with many tool invocations.