fix: match text and image attachment heights in conversation timeline (#23593)

## Problem

Text attachments (`InlineTextAttachmentButton`) and image thumbnails
(`ImageThumbnail`) rendered at different heights when displayed side by
side in user messages. Text cards had no explicit height
(content-driven), while images used `h-16` (64px).

## Changes

**`ConversationTimeline.tsx`**
- Added `h-16` to `InlineTextAttachmentButton` to match `ImageThumbnail`
- Added `isPlaceholder` prop: when the content hasn't been fetched yet
(file_id path), renders "Pasted text" in sans-serif `text-sm` with
`items-center` alignment instead of monospace `text-xs`
- Once real content loads, it still renders in `font-mono text-xs` with
`formatTextAttachmentPreview()`

**`ConversationTimeline.stories.tsx`**
- Added `UserMessageWithMixedAttachments` story showing a text
attachment and image side by side as a visual regression guard

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

@@ -299,6 +299,42 @@ export const UserMessageWithTextAttachmentOnly: Story = {
 	},
 };
 
+/** Visual regression: text and image attachments render at the same height. */
+export const UserMessageWithMixedAttachments: Story = {
+	args: {
+		...defaultArgs,
+		parsedMessages: parseMessagesWithMergedTools([
+			{
+				...baseMessage,
+				id: 1,
+				role: "user",
+				content: [
+					{ type: "text", text: "Here is a screenshot and some context" },
+					{
+						type: "file",
+						media_type: "image/png",
+						data: TEST_PNG_B64,
+					},
+					{
+						type: "file",
+						file_id: "storybook-test-text",
+						media_type: "text/plain",
+					},
+				],
+			},
+		]),
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		const images = canvas.getAllByRole("img", { name: "Attached image" });
+		expect(images).toHaveLength(1);
+		const textButtons = await canvas.findAllByRole("button", {
+			name: "View text attachment",
+		});
+		expect(textButtons).toHaveLength(1);
+	},
+};
+
 /** Text-only messages must not produce spurious image thumbnails. */
 export const UserMessageTextOnly: Story = {
 	args: {

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

@@ -123,20 +123,26 @@ const SmoothedResponse: FC<{
 const InlineTextAttachmentButton: FC<{
 	content: string;
 	onPreview?: (content: string) => void;
-}> = ({ content, onPreview }) => {
+	isPlaceholder?: boolean;
+}> = ({ content, onPreview, isPlaceholder }) => {
 	return (
 		<button
 			type="button"
 			aria-label="View text attachment"
-			className="inline-flex max-w-sm items-start gap-2 rounded-md border-0 bg-surface-tertiary px-3 py-2 text-left transition-colors hover:bg-surface-quaternary focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-content-link"
+			className="inline-flex h-16 max-w-sm items-center gap-2 rounded-md border-0 bg-surface-tertiary px-3 py-2 text-left transition-colors hover:bg-surface-quaternary focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-content-link"
 			onClick={(e) => {
 				e.stopPropagation();
 				onPreview?.(content);
 			}}
 		>
-			<FileTextIcon className="mt-0.5 size-icon-sm shrink-0 text-content-secondary" />
-			<span className="line-clamp-2 min-w-0 font-mono text-xs text-content-secondary">
-				{formatTextAttachmentPreview(content)}
+			<FileTextIcon className="size-icon-sm shrink-0 text-content-secondary" />
+			<span
+				className={cn(
+					"line-clamp-2 min-w-0 text-content-secondary",
+					isPlaceholder ? "text-sm" : "font-mono text-xs",
+				)}
+			>
+				{isPlaceholder ? content : formatTextAttachmentPreview(content)}
 			</span>
 		</button>
 	);
@@ -156,6 +162,7 @@ const TextAttachmentButton: FC<{
 	return (
 		<InlineTextAttachmentButton
 			content={content ?? "Pasted text"}
+			isPlaceholder={content === null}
 			onPreview={async () => {
 				if (content !== null) {
 					onPreview?.(content);