fix(packets): node and observer filter races (port of upstream PRs 2244 and 2242) #595

Port and adapt two upstream fixes (Kpa-clawbot/CoreScope pull request numbers 2244 and 2242; literal references, not linked):

• 2244: keep the node filter selection on a slow click or a late search result;
• 2242: re-fetch when the observer filter changes.

Check whether our Packets page has the same races. Fix them with E2E tests that reproduce them (delayed responses via route interception, no sleeps).