Replace deprecated PDESystem .indvars/.depvars with qualified accessor calls

Use ModelingToolkit.get_ivs() and ModelingToolkit.get_dvs() instead of the
deprecated pde_system.indvars and pde_system.depvars property accessors.
The accessors must be module-qualified because NeuralPDE's selective import
(using ModelingToolkit: ModelingToolkit, PDESystem, ...) does not bring
get_ivs/get_dvs into the module namespace.

Fixes all three call sites: symbolic_discretize in discretize.jl and both
get_loss_function_neural_adapter methods in neural_adapter.jl.

Supersedes #1034 which used unqualified get_ivs/get_dvs causing
UndefVarError in all PDE-related tests.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

src/discretize.jl

@@ -416,7 +416,7 @@ function SciMLBase.symbolic_discretize(pde_system::PDESystem, discretization::Ab
 
     depvars, indvars, dict_indvars,
         dict_depvars, dict_depvar_input = get_vars(
-        pde_system.indvars, pde_system.depvars
+        ModelingToolkit.get_ivs(pde_system), ModelingToolkit.get_dvs(pde_system)
     )
 
     if init_params === nothing

src/neural_adapter.jl

@@ -40,7 +40,8 @@ function get_loss_function_neural_adapter(
     eqs isa Array || (eqs = [eqs])
     domains = pde_system.domain
 
-    _, _, dict_indvars, dict_depvars = get_vars(pde_system.indvars, pde_system.depvars)
+    _, _, dict_indvars, dict_depvars = get_vars(
+        ModelingToolkit.get_ivs(pde_system), ModelingToolkit.get_dvs(pde_system))
 
     eltypeθ = recursive_eltype(init_params)
     bound = get_bounds_(domains, eqs, eltypeθ, dict_indvars, dict_depvars, strategy)
@@ -54,7 +55,8 @@ function get_loss_function_neural_adapter(
     eqs isa Array || (eqs = [eqs])
     domains = pde_system.domain
 
-    _, _, dict_indvars, dict_depvars = get_vars(pde_system.indvars, pde_system.depvars)
+    _, _, dict_indvars, dict_depvars = get_vars(
+        ModelingToolkit.get_ivs(pde_system), ModelingToolkit.get_dvs(pde_system))
 
     eltypeθ = recursive_eltype(init_params)
     lb, ub = get_bounds_(domains, eqs, eltypeθ, dict_indvars, dict_depvars, strategy)