Fix DynamicQuantities test failures in extension refactor

- Remove duplicate _is_dimension_error definition from model_parsing.jl
- Fix qualified function call in generated code for _is_dimension_error
- Add proper oneunit function for DynamicQuantities support
- Export oneunit from main module for test compatibility
- Fix get_unit fallback to return oneunit for consistent behavior

This addresses the CI failures in DynamicQuantities tests where:
1. _is_dimension_error was not accessible in generated code
2. oneunit function was missing from exports
3. get_unit was returning computed values instead of unit structure

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/ModelingToolkit.jl

@@ -296,7 +296,7 @@ export map_variables_to_equations
 
 export toexpr, get_variables
 export simplify, substitute
-export get_unit, validate, equivalent, screen_unit, get_literal_unit
+export get_unit, validate, equivalent, screen_unit, get_literal_unit, oneunit
 export build_function
 export modelingtoolkitize
 export generate_initializesystem, Initial, isinitial, InitializationProblem

src/systems/model_parsing.jl

@@ -892,8 +892,7 @@ end
 
 # Unitful convert_units functions moved to ModelingToolkitUnitfulExt extension
 
-# Extensible dimension error check - extensions can add methods
-_is_dimension_error(e) = false
+# Dimension error check function is defined in unit_check.jl
 
 convert_units(::DynamicQuantities.Quantity, value::Num) = value
 
@@ -912,7 +911,7 @@ function parse_variable_arg(dict, mod, arg, varclass, kwargs, where_types)
                     try
                         $setdefault($vv, $convert_units($unit, $name))
                     catch e
-                        if isa(e, $(DynamicQuantities.DimensionError)) || (_is_dimension_error(e))
+                        if isa(e, $(DynamicQuantities.DimensionError)) || ($(ModelingToolkit)._is_dimension_error(e))
                             error("Unable to convert units for \'" * string(:($$vv)) * "\'")
                         elseif isa(e, MethodError)
                             error("No or invalid units provided for \'" * string(:($$vv)) *

src/systems/unit_check.jl

@@ -61,6 +61,11 @@ end
 const unitless = DQ.Quantity(1.0)
 get_literal_unit(x) = screen_unit(something(__get_literal_unit(x), unitless))
 
+# Get unit value of a quantity (oneunit functionality)
+oneunit(x::DQ.AbstractQuantity) = DQ.Quantity(1.0, DQ.dimension(x))
+oneunit(x::Real) = unitless
+oneunit(x) = get_unit(x)
+
 """
 Find the unit of a symbolic item.
 """
@@ -79,8 +84,8 @@ function get_unit(op, args) # Fallback
     unit_args = get_unit.(args)
     try
         result = op(unit_args...)
-        # For operations that return a unit directly, return it
-        return screen_unit(result)
+        # For operations that return a unit directly, return oneunit to get the unit structure
+        return oneunit(result)
     catch
         try
             # Try with oneunit for numeric operations
@@ -174,7 +179,8 @@ function get_unit(x::Symbolic)
     end
 end
 
-# Add DQ.DimensionError method to existing _is_dimension_error function
+# Dimension error detection function - extensible for different unit systems
+_is_dimension_error(e) = false  # Default fallback
 _is_dimension_error(e::DQ.DimensionError) = true
 
 """