fix(markdown): reset list item font size to md (#123069)

Autofix root-cause lists were hard to read because tight Markdown list
items only render inline tokens (no nested `<p>`), so they inherited
smaller parent type instead of body size.

`DefaultListItem` now matches `DefaultParagraph` with an explicit `md`
size. That is the shared Markdown default, so it applies to every
`Markdown` / `SeerMarkdown` consumer — not an Autofix-only override.

<!-- junior-request-attribution:start -->
Requested by **nate**.
<!-- junior-request-attribution:end -->

<!-- junior-session-footer:start -->
<!-- junior-conversation-id:slack%3AC0B22JBG2N7%3A1787949252.627479 -->

--

[View Junior
Session](https://junior-prod.sentry.dev/conversations/slack%3AC0B22JBG2N7%3A1787949252.627479)
[[Sentry]](https://sentry.sentry.io/explore/conversations/slack%3AC0B22JBG2N7%3A1787949252.627479/?project=4510944073809921)

<!-- junior-session-footer:end -->

Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Nate Moore <nate.moore@sentry.io>
Co-authored-by: Nate Moore <natemoo-re@users.noreply.github.com>