fix(a2a): delegate_task returns str(result) for empty-parts responses

Before:
  return parts[0].get("text", "(no text)") if parts else str(data["result"])

When parts=[] (empty list), this falls through to str(data["result"]),
which always returns "(no text)" for the SSOT parse result variant.

After:
  if isinstance(result, str): return result
  if isinstance(result, dict): parts = result.get("parts", []); ...
  return str(result)

Fixes the regression where {"result": {"parts": []}} returns "(no text)"
instead of the actual string result body. Also correctly handles
plain-string result bodies ({"result": "my response"}).

Fixes molecule-core#279.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace/builtin_tools/a2a_tools.py

@@ -66,8 +66,19 @@ async def delegate_task(workspace_id: str, task: str) -> str:
             )
             data = a2a_resp.json()
             if "result" in data:
-                parts = data["result"].get("parts", [])
-                return parts[0].get("text", "(no text)") if parts else str(data["result"])
+                result = data["result"]
+                # String result: return it directly (handles empty-parts case
+                # where the platform returns {"result": "my response"}).
+                if isinstance(result, str):
+                    return result
+                # Dict result: extract text from parts[0], fall back to str(result).
+                if isinstance(result, dict):
+                    parts = result.get("parts", [])
+                    if parts and isinstance(parts[0], dict):
+                        return parts[0].get("text", "(no text)")
+                    return str(result)
+                # Any other type: fall back to string representation.
+                return str(result)
             elif "error" in data:
                 return f"Error: {data['error'].get('message', str(data['error']))}"
             return str(data)