[devtools] add correct scrollbar to dialog body (#80776)
Since the scrollbar is applied to the whole dialog by default, the
header and the footer, which are children of the dialog, were also
included in the overflow. Therefore, this PR ensures that the DevTools
panel's dialog only targets the dialog body to have the overflow.
### Before
https://github.com/user-attachments/assets/82722635-6f8c-4608-9b73-9e60090dbbfe
### After
https://github.com/user-attachments/assets/0fb4e9a2-bac6-493b-96bf-fec796a15455
