Fix: Replace ODESystem with System in tests for ModelingToolkit v10 compat

[ChrisRackauckas-Claude]

Summary

Fix the Downgrade CI test failure by replacing ODESystem with System in tests.

Problem

The Downgrade CI workflow runs tests with minimum compatible dependency versions (ModelingToolkit v10). In these older versions, ODESystem is not exported/available, causing test failures:

UndefVarError: `ODESystem` not defined

at test/test_julia_parser.jl:66 and test/test_antlr_parser.jl:44.

Changes

• test/test_julia_parser.jl:66: Changed ODESystem to System
• test/test_antlr_parser.jl:44: Changed ODESystem to System

Test plan

• All tests pass locally with Pkg.test()
• Downgrade CI passes with this fix
• Regular Tests CI passes

Notes

The Format Check CI is also failing due to formatting issues in the codebase, but addressing formatting is outside the scope of this fix (and was explicitly excluded from the CI health check scope).

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

CI Results

✅ All relevant tests now pass!

Check	Status
test (Core, alldeps, 1.10)	✅ PASS - This was the failing test that this PR fixes
Tests (1, Core)	✅ PASS
Tests (1, Quality)	✅ PASS
Tests (lts, Core)	✅ PASS
Tests (lts, Quality)	✅ PASS
Tests (pre, Core)	✅ PASS
Tests (pre, Quality)	✅ PASS
Documentation	✅ PASS
Spell Check	✅ PASS
Format Check	❌ FAIL - Pre-existing issue, unrelated to this fix

The Format Check failure is a pre-existing issue in the codebase (several files have formatting that doesn't match JuliaFormatter SciMLStyle). This should be addressed in a separate PR focused on formatting, as it affects multiple files:

• grammar/README.md
• src/ast.jl
• src/evaluator.jl
• src/julia_parser.jl
• src/scratch.jl
• test/test_antlr_parser.jl
• test/test_error_messages.jl
• test/test_julia_parser.jl

This PR is ready for review and merge.