Fix : TypeError in IntegralAffineCurve.function_field() for non-default variable names

[PrafulVRaj]

Summary

When AffineSpace is constructed with names other than the default (x, y), simple_model()/separable_model() rename the base-field generator (e.g. y to y_) to avoid a collision in the extension tower. The Horner-scheme evaluator in _nonsingular_model used .x on function-field elements, which bypasses coercion and returns values in Frac(k[y]) rather than in FR = Frac(k[y,z]). Sage has no registered coercion between these two differently-named fraction fields, so the accumulation fails with TypeError.

Fix

Build an explicit ring homomorphism base_to_FR from M.base_field() to FR before entering the evaluate closure, and replace ._x with base_to_FR(). This routes every coefficient through Sage's public coercion machinery and guarantees it lands in FR regardless of generator naming.

Fixes #41643

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