fix(notebook): restore the %%backtest cell magic (shadowed by the same-named line magic)

[Anai-Guo]

The bug

BacktestMagics (investing_algorithm_framework/notebook/magic.py) defines backtest twice in the same class body:

@cell_magic
def backtest(self, line, cell):   # %%backtest -- inline strategy
    ...

@line_magic
def backtest(self, line):  # noqa: F811   # %backtest -- strategy file
    ...

Both IPython decorators call record_magic(magics, kind, magic_name, method_name) with the same method name, so the class-level table ends up as

{'line': {'backtest': 'backtest'}, 'cell': {'backtest': 'backtest'}}

and the class dict keeps only the last definition. Magics.__init__ then resolves both entries with getattr(self, 'backtest') — i.e. the line-magic body, whose signature is (self, line).

run_cell_magic calls the registered function as fn(line, cell), so the documented %%backtest cell magic (module docstring, README-style example in its own docstring) cannot run at all:

TypeError: BacktestMagics.backtest() takes 2 positional arguments but 3 were given

The # noqa: F811 silenced the one linter that would have flagged it; flake8 stays quiet, so this has been invisible.

Verification

I executed the real BacktestMagics class body (extracted with ast from main and from this branch) against real IPython 9.17.1 Magics/magics_class, instantiated it, and called through the actual magic registry — helpers (_build_parser, _run_backtest, …) stubbed so no market data is needed:

== BEFORE (main) ==
  registry: {'line': ['backtest'], 'cell': ['backtest']}
  line magic -> 'BACKTEST_OBJ'
  cell magic -> TypeError: BacktestMagics.backtest() takes 2 positional arguments but 3 were given
== AFTER  (patched) ==
  registry: {'line': ['backtest'], 'cell': ['backtest']}
  line magic -> 'BACKTEST_OBJ'
  cell magic -> 'BACKTEST_OBJ'

The fix

Collapse the pair into a single @line_cell_magic dispatcher — the supported way for one magic class to serve both %name and %%name — that forwards to the two original bodies, kept verbatim as _backtest_cell / _backtest_line:

@line_cell_magic
def backtest(self, line, cell=None):
    if cell is None:
        return self._backtest_line(line)
    return self._backtest_cell(line, cell)

%backtest strategies/my_strategy.py --start ... behaves exactly as before; %%backtest now reaches its own implementation. The noqa: F811 is no longer needed. The no-IPython fallback stub block is updated to match the new import.

flake8 --isolated --max-line-length 79 --select=E,W,F reports the same two pre-existing W503 lines before and after — nothing introduced.

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