Use %% in the @magic_arguments usage line of cell magics 🤖🤖 - #15432

🤖🤖 Fixes #14715.

The usage line built by @magic_arguments() always started with %, because MagicHelpFormatter.add_usage hard-codes that prefix. So the help of cell magics showed the line-magic form:

In [1]: %%capture?
::

  %capture [--no-stderr] [--no-stdout] [--no-display] [output]

The same happens for %%writefile, %%html and %%script.

The parser is built when @magic_arguments() runs, and at that point it has no way to know the magic kind. In the documented decorator order, @cell_magic / @register_cell_magic sit under @magic_arguments() and run first. So:

• The _method_magic_marker and _function_magic_marker decorators now record the kind on the function, as _ipython_magic_kind (MAGIC_KIND_ATTR, next to the existing _ipython_magic_* attributes).
• construct_parser uses a new CellMagicHelpFormatter, which uses the %% prefix, when the kind is "cell". A formatter_class passed through @kwds still wins.
• Line and line/cell magics keep %, so %time, %timeit and %code_wrap don't change.

After the change:

  %%capture [--no-stderr] [--no-stdout] [--no-display] [output]
  %%writefile [-a] filename
  %%html [--isolated]
  %time [--no-raise-error]

If @cell_magic is applied above @magic_arguments(), the parser has already been built and the help keeps %. No built-in magic uses that order.

Separately, %%script? shows %%shebang because @cell_magic("script") names the magic differently from the method, and real_name() uses the function name. I left that out to keep this PR to the prefix.

The new test in tests/test_magic_arguments.py covers a line magic, a cell magic (both the bare and the named @cell_magic("...") form) and a line/cell magic.

🤖 Generated with Claude Code