Internal helper to construct the right-hand side of a fixed-effects formula from factors, interaction flag, and continuous covariates.
build_fixed_rhs(
factors = NULL,
factor_interaction = FALSE,
continuous_covariates = NULL,
data = NULL
)A character string representing the RHS (e.g., "~ 1 + dose + drug").
Character vector of factor names (can be NULL).
Logical. If TRUE and two factors provided, include their interaction.
Character vector of continuous covariate names.