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.

The issue is in frontend/src/script.ts, in the socket.on('agent_response', ...) listener's case 'message' handler. Look at the existing processPart helper function to see how it handles data parts. The fix is to iterate over event.parts and call processPart for each, then combine and render the content. Test by connecting to an A2A v1.0 agent that returns a message with data parts and verifying the JSON appears in the chat UI.

索引モデルが issue の本文から書いたものです。