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

[dgwartney]

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:

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 <pre><code> 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:

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 = `<span class="kind-chip kind-chip-message">${event.kind}</span> ${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.