Add sign_magneticfield validation to exchange_legendre.py

[Copilot]

Missing input validation for sign_magneticfield parameter in get_exchange_kernels_GaussLegendre. The parameter was used without checking it's a valid value (1 or -1).

Changes

• Add validation check matching the pattern in planewave.py and exchange_hankel.py

if sign_magneticfield not in (1, -1):
    raise ValueError("sign_magneticfield must be 1 or -1")

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[Copilot]

Pull request overview

This PR adds input validation for the sign_magneticfield parameter in get_exchange_kernels_GaussLegendre to ensure it only accepts values of 1 or -1, bringing it into consistency with the validation patterns already established in planewave.py and exchange_hankel.py.

• Adds defensive validation check for the sign_magneticfield parameter
• Uses identical validation pattern and error message as other modules
• Places validation in the "Input handling" section before parameter usage

Comments suppressed due to low confidence (1)

src/quantumhall_matrixelements/exchange_legendre.py:107

• Variable Veff is not used.

        Veff = None

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