Explore: Make content outline more responsive

Requested in [Slack](https://raintank-corp.slack.com/archives/C02TXB92G6T/p1759504088490029). When Content Outline is expanded but there's no scroll the outline takes spaces and looks annoying. We could collapse the outline >if the content height is less than the screen height<. We need to be careful with UX and code complexity here. We already keep the state if the Outline is on/off + if it's expanded or collapsed. We also have compact mode which controls how outline is shown. Adding another condition can make the UI more confusing (outline may expand/collapse after getting results). The code may also get more complex if add additional condition/state management. Might be worth revisiting Content Outline interactions as a whole and maybe simplify it.