Studio: stop hidden source badges from inflating chat scroll height (#5822)

The SourcesGroup measurement container renders every citation badge
off-screen with `invisible absolute` so we can measure how many fit in
two rows. Absolute descendants still contribute to the parent's
scrollable overflow region, so long source lists added hundreds of
pixels of phantom scroll space below the assistant message that the
user could scroll into.

Wrap the measurement container in an absolute, `h-0`, `overflow-hidden`
box so the off-screen pills are clipped out of the scrollable overflow
region. Measurement still works because offsetTop is read relative to
the positioned wrapper.