Message-kind responses with data parts are not rendered in chat

## Summary

The chat UI does not render agent responses when the response is a `message`-kind event containing `data` parts (structured JSON). The message is received by the inspector but nothing is displayed.

## Root Cause

In `frontend/src/script.ts`, the `case 'message'` handler inside the `socket.on('agent_response', ...)` listener only looks for `text` parts:

```typescript
const textPart = event.parts?.find(p => p.text);
if (textPart && textPart.text) {
// render text
}
```

If all parts are `data` parts (i.e. `kind: 'data'`, `data: {...}`), `textPart` is `undefined` and nothing is rendered — the message arrives but is silently dropped in the UI.

The `processPart` helper function already handles `data` parts correctly (renders as formatted `

` JSON), but it is only called in the `task`, `status-update`, and `artifact-update` cases — not in the `message` case.


## Steps to Reproduce

1. Connect the inspector to an A2A v1.0 agent that returns an **immediate Message response** (i.e. uses the synchronous workflow — enqueues a `Message` object rather than a `Task` with artifacts)

2. Send any message

3. The agent's response arrives (visible in the Debug Console) but the chat window remains empty

## Expected Behaviour

Structured data returned in a `message`-kind response should be rendered as formatted JSON in the chat window, consistent with how `data` parts are rendered in `artifact-update` responses.

## Proposed Fix

Replace the text-only lookup with a loop over all parts using the existing `processPart` helper:

```typescript

case 'message': {

  const allContent: string[] = [];

  event.parts?.forEach(p => {

    const content = processPart(p);

    if (content) allContent.push(content);

  });

  if (allContent.length > 0) {

    const combinedContent = allContent.join('');

    const messageHtml = `${event.kind} ${combinedContent}`;

    appendMessage('agent', messageHtml, displayMessageId, true, validationErrors);

  }

  break;

}

```

This renders `text`, `data`, and `file` parts in message responses using the same logic already used for artifacts.

## Context

Discovered while validating a Google Maps A2A v1.0 agent (`a2a-sdk` based) that uses the immediate Message response pattern for synchronous skill calls.