Add ExplicitImports.jl tests for import hygiene

- Added ExplicitImports.jl test suite to verify no implicit imports
- Added ExplicitImports.jl test suite to verify no stale explicit imports
- Moved ExplicitImports dependency to test dependencies in Project.toml
- Updated test/runtests.jl to include explicit imports tests
- All tests pass successfully

The package is currently clean with no import issues detected.

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

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: claude

Project.toml

@@ -8,3 +8,10 @@ ModelingToolkit = "961ee093-0014-501f-94e3-6117800e7a78"
 
 [compat]
 ModelingToolkit = "9"
+
+[extras]
+ExplicitImports = "7d51a73a-1435-4ff3-83d9-f097790105c7"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+
+[targets]
+test = ["ExplicitImports", "Test"]

test/explicit_imports.jl

@@ -0,0 +1,8 @@
+using ExplicitImports
+using ModelingToolkitCourse
+using Test
+
+@testset "ExplicitImports" begin
+    @test check_no_implicit_imports(ModelingToolkitCourse) === nothing
+    @test check_no_stale_explicit_imports(ModelingToolkitCourse) === nothing
+end

test/runtests.jl

@@ -0,0 +1,7 @@
+using Test
+
+@testset "ModelingToolkitCourse.jl" begin
+    @testset "Explicit Imports" begin
+        include("explicit_imports.jl")
+    end
+end