add assay-specific labels to plot_predictive_curve.R

[kaiemjoy]

pull Y axis labels for the plot depending on the assay units (ELISA OD, Kinetic ELISA, multiplex-bg)

[Kwan-Jenny]

@copilot Please revise plot_predictive_curve() in R/plot_predictive_curve.R so that the Y-axis label is assay-specific. Add a new optional argument assay (or assay_type) with default NULL. If assay is provided, use it to determine the Y-axis label; if it is NULL, attempt to infer the assay from the input data (e.g., columns named assay, assay_type, or assay_units), and otherwise fall back to the current default label. Implement a small internal helper that maps assay types (case-insensitive) as follows: ELISA_OD → "Optical density (OD)", Kinetic_ELISA → "Kinetic ELISA signal", and multiplex-bg → "MFI (background-subtracted)"; if an unsupported assay is provided, issue a warning and keep the existing label. Update the ggplot call to set the Y-axis label via labs(y = ...) without breaking existing behavior. Then update all roxygen2 documentation for plot_predictive_curve() to document the new argument and describe the inference and fallback behavior. Add or update testthat tests to verify that the returned ggplot object has the correct y label for each supported assay and that an unknown assay triggers a warning while preserving the default label. Finally, update any relevant examples, vignettes, or README code that call plot_predictive_curve() to include at least one example demonstrating assay-specific labeling, and ensure all tests and examples pass R CMD check.