fix(site): add bottom spacing for no-renderable assistant fallback messages (#24551)

closes CODAGT-125

Assistant messages that show only the fallback text ("Message has no
renderable content.") were missing bottom spacing before the next user
bubble, because `needsAssistantBottomSpacer` only covered reasoning-only
and sources-only cases.

Extend the spacer predicate to also trigger when
`!hasRenderableContent`, and add a `data-testid` to the spacer element
for testability. A new Storybook story
(`NoRenderableContentFallbackSpacing`) covers this regression.

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

@@ -1391,3 +1391,32 @@ export const SourcesOnlyAssistantSpacing: Story = {
 		).toBeInTheDocument();
 	},
 };
+
+export const NoRenderableContentFallbackSpacing: Story = {
+	args: {
+		...defaultArgs,
+		parsedMessages: buildMessages([
+			{
+				...baseMessage,
+				id: 101,
+				role: "assistant",
+				content: [],
+			},
+			{
+				...baseMessage,
+				id: 102,
+				role: "user",
+				content: [{ type: "text", text: "Thanks for trying!" }],
+			},
+		]),
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		expect(
+			canvas.getByText("Message has no renderable content."),
+		).toBeInTheDocument();
+		expect(
+			document.querySelector('[data-testid="assistant-bottom-spacer"]'),
+		).toBeInTheDocument();
+	},
+};

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

@@ -499,10 +499,9 @@ const ChatMessageItem = memo<{
 							)}
 						</div>
 					)}
-				{/* Spacer for assistant messages without an action bar
-				   (e.g. reasoning-only or sources-only) so they have
-				   consistent bottom padding before the next user bubble. */}
-				{displayState.needsAssistantBottomSpacer && <div className="min-h-6" />}
+				{displayState.needsAssistantBottomSpacer && (
+					<div className="min-h-6" data-testid="assistant-bottom-spacer" />
+				)}
 				{previewImage && (
 					<ImageLightbox
 						src={previewImage}

site/src/pages/AgentsPage/components/ChatConversation/messageHelpers.ts

@@ -58,11 +58,17 @@ export const deriveMessageDisplayState = ({
 		userInlineContent.length > 0 || Boolean(parsed.markdown.trim());
 	const hasFileBlocks = userFileBlocks.length > 0;
 	const hasCopyableContent = Boolean(parsed.markdown.trim());
+	const hasRenderableContent =
+		parsed.blocks.length > 0 ||
+		parsed.tools.length > 0 ||
+		parsed.sources.length > 0;
 	const needsAssistantBottomSpacer =
 		!hideActions &&
 		!isUser &&
 		!hasCopyableContent &&
-		(Boolean(parsed.reasoning) || parsed.sources.length > 0);
+		(Boolean(parsed.reasoning) ||
+			parsed.sources.length > 0 ||
+			!hasRenderableContent);
 	const hasToolResultsOnly =
 		parsed.toolResults.length > 0 &&
 		parsed.toolCalls.length === 0 &&