fix(site/src/pages/AgentsPage): cap queued messages list height so chat scroll keeps working (#24950)

Linear:
[CODAGT-313](https://linear.app/codercom/issue/CODAGT-313/unable-to-scroll-long-queued-messages-in-coder-agents)

## Summary

When many messages are queued in the agent chat, the chat history
becomes unscrollable: mouse wheel and scrollbar drag both stop
responding.

The input wrapper in `AgentChatPageView.tsx:496` is `shrink-0
overflow-y-auto` with **no `max-height`**, so `overflow-y-auto` is a
no-op and the section grows unbounded as `QueuedMessagesList` adds rows.
Its sibling `ChatScrollContainer` is `flex-1 min-h-0`, so it absorbs the
shrinkage and `clientHeight` collapses to 0. The chat list is then a
zero-height viewport with nothing to scroll.

Measured against the actual `AgentChatPageView` rendered in Storybook
with 20 queued messages (1280x800):

| | scroll-container `clientHeight` | input wrapper height | scrollable?
|
|---|---:|---:|---|
| 0 queued | 502 px | 270 px | yes |
| 20 queued, `main` | **0 px** | 1182 px | **no** |
| 20 queued, this PR | 258 px | 502 px | yes |

## Demo

![scroll fix
side-by-side](https://raw.githubusercontent.com/coder/coder/bpmct/codagt-313-assets/scroll-fix-side-by-side.gif)

Left (`main`): wheel-up does nothing because the chat scroll container
has been crushed to zero height.
Right (this PR): the queued list scrolls inside its own pane and the
chat history scrolls normally.

Recording is `AgentChatPageView` rendered through Storybook with the
production component source. The same gesture (wheel-up over the chat
history, then wheel-down over the queued list) is applied to both sides.
Source for the recording is in `bpmct/codagt-313-assets`.

## Change

```diff
-		<div className={cn("flex w-full flex-col", className)}>
+		// Cap the queue at ~40% of the small viewport so a long queue
+		// does not push the chat history's scroll container down to
+		// zero height (CODAGT-313). The list scrolls inside its own pane.
+		<div
+			className={cn(
+				"flex w-full flex-col max-h-[40svh] overflow-y-auto [scrollbar-gutter:stable] [scrollbar-width:thin] [scrollbar-color:hsl(var(--surface-quaternary))_transparent]",
+				className,
+			)}
+		>
```

## Why this spot, not the outer wrapper

The composer textarea already self-caps at `max-h-[50vh]` in
`ChatMessageInput.tsx:688`, so the only unbounded growth source in the
input section is the queued list. Capping the list keeps the constraint
colocated with the component that owns it, and any future consumer of
`QueuedMessagesList` is automatically safe.

`40svh` (small viewport height) so the queue doesn't fight with the iOS
keyboard once it appears, matching the `h-dvh` decision in #24848.

---

*Generated by Coder Agents.*

site/src/pages/AgentsPage/components/QueuedMessagesList.tsx

@@ -170,7 +170,12 @@ export const QueuedMessagesList: FC<QueuedMessagesListProps> = ({
 	const isBusy = busyItem !== null;
 
 	return (
-		<div className={cn("flex w-full flex-col", className)}>
+		<div
+			className={cn(
+				"flex w-full flex-col max-h-[40svh] overflow-y-auto [scrollbar-gutter:stable] [scrollbar-width:thin] [scrollbar-color:hsl(var(--surface-quaternary))_transparent]",
+				className,
+			)}
+		>
 			{visibleItems.map((item, index) => {
 				const isEditing = item.id === editingMessageID;
 				const isFirst = index === 0;