Strip leading whitespace from code before evaluating

If auto-indent is turned on and you're inside of a statement in the buffer, python code that you write will be indented. However, leading whitespace a the beginning of python code is always an error. We should probably strip leading whitespace from code before evaluating it.

Start by tracing the plugin's code-evaluation path and how auto-indent affects code inside a statement. Reproduce the leading-whitespace case, then verify that evaluating indented Python succeeds without changing other evaluation behavior.

Written by the indexing model from the issue text.