Add expression variable collector

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Summary

• add ModelingToolkitBase.collect_expression_vars! as a public helper for collecting variables from arbitrary symbolic expressions
• delegate classification to MTKBase's existing collect_var! logic so downstream packages do not need to duplicate parameter/unknown handling
• document the helper and cover parameter, unknown, independent-variable, and default-expression collection behavior

Motivation

Catalyst needs to collect variables from expression leaves while preserving MTKBase's parameter/unknown classification behavior. Exposing this helper keeps that classification logic in MTKBase instead of copying it downstream.

Validation

• julia +1.10 --project=lib/ModelingToolkitBase -e 'cd("lib/ModelingToolkitBase/test"); include("variable_utils.jl")' passed\n- julia +1.10 --project=lib/ModelingToolkitBase -e 'using ModelingToolkitBase; @assert isdefined(ModelingToolkitBase, :collect_expression_vars!); println(ModelingToolkitBase.collect_expression_vars!)' passed\n- Runic check passed on edited Julia files\n- git diff --check passed\n- GROUP=ModelingToolkitBase_InterfaceII julia +1.10 --project=lib/ModelingToolkitBase -e 'using Pkg; Pkg.test()' did not pass: InterfaceII reported 7153 passed, 2 errored, 4 broken, with both errors in Optimal Control + Constraints Tests BVP cases (Lotka-Volterra and Cost function compilation). The targeted Variable Utils Test section in that run passed with 139 tests.

[ChrisRackauckas-Claude]

CI triage note from local follow-up:

• Runic / Runic Format Check fails on untouched files (lib/ModelingToolkitBase/src/systems/codegen.jl, index_cache.jl, src/initialization.jl, test/initialization_jacobian_analysis.jl). The edited Julia files passed Runic locally.
• Spell Check with Typos reports existing Lamba/lambda annotations in untouched files.
• docs build fails during dependency resolution before docs build (Optim/OptimizationOptimJL conflict).
• Downgrade / Downgrade Tests - InterfaceI fails during dependency resolution on OrdinaryDiffEqCore.
• The Catalyst checks relevant to the original LTS modeling issue were still pending at this point.

Local targeted validation remains:

• julia +1.10 --project=lib/ModelingToolkitBase -e 'cd("lib/ModelingToolkitBase/test"); include("variable_utils.jl")' passed.\n- ModelingToolkitBase.collect_expression_vars! is defined and reachable in the MTKBase environment.

[AayushSabharwal]

Why is this necessary when collect_vars! exists?