fix(site): update sticky messages during streaming (#23577)

## Problem

The sticky user message visual state (`--clip-h`, fade gradient, push-up
positioning) is driven by an `update()` function that only ran on
`scroll` events. The chat scroll container uses `flex-col-reverse`,
where `scrollTop = 0` means "at bottom." When streaming content grows
the transcript while the user is auto-scrolled to the bottom,
`scrollTop` stays at `0` — no `scroll` event fires — so `update()` never
runs and the sticky messages become visually stale until the user
manually scrolls.

## Fix

Add a `ResizeObserver` on the scroller's content wrapper inside the
existing `useLayoutEffect` that sets up the scroll/resize listeners.
When the content wrapper resizes (streaming growth), it fires the
observer which calls `update()` through the same RAF-throttle pattern
used by the scroll handler.

Single observer per sticky message instance. Zero cost when nothing is
resizing. Cleanup handled in the same effect teardown.

site/src/pages/AgentsPage/components/AgentDetail/ConversationTimeline.tsx

@@ -856,6 +856,24 @@ const StickyUserMessage: FC<{
 			});
 		};
 
+		// Re-run the visual update when the scrollable content height
+		// changes (e.g. streaming responses growing the transcript).
+		// In flex-col-reverse, scrollTop stays at 0 when pinned to
+		// bottom so no scroll event fires — but the content wrapper
+		// resizes and this observer catches that.
+		const contentEl = scroller.firstElementChild as HTMLElement | null;
+		let contentRafId: number | null = null;
+		const contentObserver = contentEl
+			? new ResizeObserver(() => {
+					if (contentRafId !== null) return;
+					contentRafId = requestAnimationFrame(() => {
+						contentRafId = null;
+						update();
+					});
+				})
+			: null;
+		contentObserver?.observe(contentEl!);
+
 		scroller.addEventListener("scroll", onScroll, { passive: true });
 		window.addEventListener("resize", onResize);
 		update();
@@ -866,8 +884,10 @@ const StickyUserMessage: FC<{
 		return () => {
 			scroller.removeEventListener("scroll", onScroll);
 			window.removeEventListener("resize", onResize);
+			contentObserver?.disconnect();
 			container.style.removeProperty("--overlay-ready");
 			if (rafId !== null) cancelAnimationFrame(rafId);
+			if (contentRafId !== null) cancelAnimationFrame(contentRafId);
 		};
 	}, []);