Exception list now follows canonical ordering spec

Minor changes

• 5535ecd fix(errors): emit $exception_list in canonical order — index 0 is the caught/outermost exception, causes follow in unwrap order, and the root cause is last (previously the list was reversed with the root cause first). This aligns posthog-python with the cross-SDK exception ordering spec. Frame order within each stacktrace is unchanged. — Thanks @cat-ph!