feat: display tool execution errors in chat UI

Add error message rendering to MessageBubble component so users can see
detailed error messages when tools fail (e.g., missing API keys).

Previously only showed "output-error" badge without the actual error text.
Now displays the error message in a red-tinted box below the tool call.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/playground/src/components/chat/MessageBubble.tsx

@@ -92,6 +92,18 @@ export function MessageBubble({
                           </pre>
                         </div>
                       )}
+
+                      {/* Tool Error */}
+                      {toolPart.error && (
+                        <div>
+                          <div className="mb-1 text-xs font-semibold text-red-500">Error:</div>
+                          <pre className="overflow-x-auto rounded bg-red-500/10 p-2 text-xs text-red-400">
+                            {typeof toolPart.error === 'string'
+                              ? toolPart.error
+                              : JSON.stringify(toolPart.error, null, 2)}
+                          </pre>
+                        </div>
+                      )}
                     </div>
                   );
                 }