add FeRx implementation support to model objects

[JordanBrooks33]

Summary

Adds FeRx implementation support to PKPDsim model objects, mirroring the existing NONMEM implementation pattern.

This change enables models to carry FeRx code as an attribute, allowing downstream tools (e.g., irxmodval) to access and utilize FeRx implementations for model validation and translation workflows.

Changes

R/model_from_api.R

• Added ferx parameter to model_from_api() function signature
• Added logic to read FeRx file contents when both ferx path and def$implementations$ferx exist
• Pass ferx to new_ode_model() call

R/new_ode_model.R

• Added ferx parameter to new_ode_model() function signature
• Added roxygen documentation for new parameter
• Added NULL/empty handling: if(is.null(ferx) || length(ferx) == 0) { ferx <- "NULL" }
• Added [FERX] placeholder to template replacement matrix

inst/template/R/model.R

• Added attr(ret, "ferx") <- "[FERX]" to store FeRx implementation as model attribute

Pattern

Follows identical pattern to existing NONMEM support:

• Parameter flows: model_from_api() → new_ode_model() → template substitution → model attribute
• Stored as string attribute on compiled model object
• Accessible via attr(model, "ferx")

Related PRs

• models: RXR-3282 (FeRx implementations added to model definitions)
• irxmodval: RXR-3282 (FeRx validation support)

Test Plan

• Verify existing NONMEM attribute behavior unchanged
• Verify ferx attribute present on models built with FeRx implementations
• Verify ferx attribute is "NULL" when no FeRx implementation provided

Associated PRs:

• https://github.com/InsightRX/irxmodval/pull/352
• https://github.com/InsightRX/models/pull/951

[jasmineirx]

lgtm, but it could be nice to add some testing around these metadata fields