Resolves#9420 (critical memory usage issue in PROD deployment)
When the plugin's ExecuteCodeTaskHandler returns a Uint8Array (e.g. from penpotUtils.exportImage),
JSON.stringify previously serialized it as an object with numeric string keys,
causing ~10x payload expansion and large peak heap usage on the server side.
The plugin now wraps a top-level Uint8Array result in a tagged envelope
{ __type: "base64", data: <base64> }, and ImageContent.byteData decodes this envelope
on the server. The legacy numeric-keyed-object path is retained as a fallback for
compatibility with older plugin builds.
This matches the behaviour of findShape, more closely aligning with
the LLM's expectations (given the lack of concrete information in
the instructions)
