Strip ANSI escape sequences from Sphinx directive output

Fixes #11424. When IPython's Sphinx directive executes code blocks,
stderr/stdout and tracebacks can contain raw ANSI escape sequences
that appear as garbled text in HTML/PDF documentation output.

This adds a regex-based ANSI stripping step to process_input in
EmbeddedSphinxShell so escape codes are removed before output
reaches the rendered documentation. Also adds unit tests for the
stripping behavior.