Dev UI chat bubble collapses multi-line node output onto one line

> Dev UI issue (prebuilt `adk-web` bundle served by `adk web`) — please transfer to google/adk-web if that is the right home.

Newlines survive on the wire and in the raw `output` panel of the same event, but the chat bubble renders them as spaces — consistent with the bubble being rendered as markdown, where a single `\n` is not a hard break. `adk run` prints all three lines correctly.

```
cd dev && npx adk web ../samples/workflows/routes -p 8231
# app=fan_out_join, send "hello world"
```

```
bubble: Uppercase: HELLO WORLD Length: 11 Reversed: dlrow olleh
raw: "Uppercase: HELLO WORLD
Length: 11
Reversed: dlrow olleh"
```

The sample's `final_task_D` exists specifically to `.join('\n')` three branch results, so column alignment is the whole point of its output.