fix(test): assert the blockquoted MCP startup transcript line (#1543)

`#1362` moved every transcript fallback for clients without
`session.notices` into its own blockquote paragraph (`\n\n> …\n\n`) and
updated the existing assertions, but missed the MCP server startup one.
`main` has been red since that merge:

- The `Build` job on `47b64c0` fails with an annotation at
`src/tests/acp-agent.test.ts` line 13860:

  ```
AssertionError: expected '\n\n> **MCP server "client-fs" failed…' to be
'**MCP server "client-fs" failed to st…'
  ```

- The neighbouring fallback tests already assert the blockquote form,
e.g. `"\n\n> **Warning:** hook says no\n\n"` and the two-line `Stop
says:` message.

This updates the expected string to the blockquoted text; product code
is untouched.

## Validation

- `npx vitest run src/tests/acp-agent.test.ts -t "falls back to a
transcript line without the notices capability"` → 1 passed
- `npx vitest run src/tests/acp-agent.test.ts` → 759 passed | 9 skipped
- `npm run check` → passes

src/tests/acp-agent.test.ts

@@ -13858,7 +13858,7 @@ describe("MCP server startup failures", () => {
     );
     expect(lines).toHaveLength(1);
     expect(lines[0].content.text).toBe(
-      '**MCP server "client-fs" failed to start:** Its tools are unavailable in this session. Run /mcp for the error.',
+      '\n\n> **MCP server "client-fs" failed to start:** Its tools are unavailable in this session. Run /mcp for the error.\n\n',
     );
   });
 });