Read full plug results in the Studio debugger

The Studio plug debugger cut every result at one line and failed with a raw traceback on some return values.

• Added an expandable result under each method: a result too long for its cell opens in full, with a copy button, and a failed call opens its whole error the same way.
• Added image previews: a method returning a PNG, JPEG, GIF or BMP image as base64 text now shows the picture, named with its format and size, in the debugger, the call log and the assistant's chat.
• Improved the error for a return value a phase cannot receive (bytes, a datetime, a numpy number): the debugger and the test run now both say the method ran, which type is the problem and how to convert it, for example x.item() for a numpy number or base64.b64encode(data).decode() for bytes.