Adds a new %xmode Doctest mode that formats tracebacks for easy copy-paste into Python doctests. The output shows only the traceback header, a literal ellipsis, and the exception line:

It will look like this when set to doctest:

Traceback (most recent call last):
    ...
ZeroDivisionError: division by zero

This is useful because Python's doctest module accepts ... as a wildcard in expected traceback output, so this format lets users copy-paste IPython output directly into doctests.

changes

• Added Doctest mode to FormattedTB.valid_modes and routing logic in ultratb.py
• Added structured_traceback_doctest method to ListTB
• Updated xmode config enum in interactiveshell.py
• Updated %xmode magic docstring in basic.py
• Added test in test_ultratb.py

• Fixed test_xmode in test_magic.py to avoid hardcoding the mode count. So previously, 5 was hardcoded in the range, so updated it to use the mode count based on the size of the list.

• Added whatsnew entry

Closes #14925