fix(agent-chat): use direct concatenation for completed message text blocks (#23009)

## Problem

LLMs stream list markers and item content as separate text blocks:

```json
{ "type": "text", "text": "Intro\n\n- " }
{ "type": "text", "text": "First item" }
{ "type": "text", "text": "\n- " }
{ "type": "text", "text": "Second item" }
```

The **streaming path** concatenated these directly → `"- First item"` ✅

The **completed path** used `appendText` which inserted `\n` between
chunks → `"- \nFirst item"` ❌

Every CommonMark parser treats `"- \nText"` (marker and content on
different lines, content not indented) as an empty `<li>` followed by a
sibling `<p>`, producing broken list rendering once a message finished
streaming.

## Fix

Make `appendText` use direct concatenation — the same as the streaming
path. The API text blocks already contain all necessary whitespace and
newlines; inserting extra `\n` between them was the bug.

## Changes

- **`messageParsing.ts`** — `appendText` simplified to direct concat
(skip whitespace-only chunks). `appendParsedTextBlock` no longer passes
a custom joiner, so it uses the same default as the streaming path.
- **`messageParsing.test.ts`** — Updated existing merge test
expectation; added regression test with the exact LLM list-marker
payload.

site/src/pages/AgentsPage/AgentDetail/messageParsing.test.ts

@@ -101,11 +101,28 @@ describe("parseMessageContent", () => {
 			{ type: "text", text: "Line one" },
 			{ type: "text", text: "Line two" },
 		]);
-		expect(result.markdown).toBe("Line one\nLine two");
+		expect(result.markdown).toBe("Line oneLine two");
 		expect(result.blocks).toHaveLength(1);
 		expect(result.blocks[0]).toEqual({
 			type: "response",
-			text: "Line one\nLine two",
+			text: "Line oneLine two",
+		});
+	});
+
+	it("normalizes list markers split across text blocks", () => {
+		// LLMs stream list markers and item content as separate text
+		// blocks. Both paths (streaming and completed) must concatenate
+		// them directly so the marker and content stay on the same line.
+		const result = parseMessageContent([
+			{ type: "text", text: "Intro\n\n- " },
+			{ type: "text", text: "First item" },
+			{ type: "text", text: "\n- " },
+			{ type: "text", text: "Second item" },
+		]);
+		expect(result.blocks).toHaveLength(1);
+		expect(result.blocks[0]).toEqual({
+			type: "response",
+			text: "Intro\n\n- First item\n- Second item",
 		});
 	});
 

site/src/pages/AgentsPage/AgentDetail/messageParsing.ts

@@ -11,15 +11,12 @@ import type {
 	RenderBlock,
 } from "./types";
 
+/** Concatenate text chunks, skipping whitespace-only values. */
 const appendText = (current: string, next: string): string => {
-	const trimmed = next.trim();
-	if (!trimmed) {
+	if (!next.trim()) {
 		return current;
 	}
-	if (!current) {
-		return next;
-	}
-	return `${current}\n${next}`;
+	return `${current}${next}`;
 };
 
 export const asOptionalTitle = (value: unknown): string | undefined =>
@@ -79,13 +76,14 @@ const emptyParsedMessageContent = (): ParsedMessageContent => ({
 	sources: [],
 });
 
-/** Wraps appendTextBlock with newline-joining for complete message blocks. */
+/** Wraps appendTextBlock using the same direct concatenation as
+ *  the streaming path so both produce identical markdown. */
 const appendParsedTextBlock = (
 	blocks: RenderBlock[],
 	type: "response" | "thinking",
 	text: string,
 	title?: string,
-): RenderBlock[] => appendTextBlock(blocks, type, text, title, appendText);
+): RenderBlock[] => appendTextBlock(blocks, type, text, title);
 
 export const ensureToolBlock = (
 	blocks: RenderBlock[],