Add ExplicitImports.jl tests to CI

[ChrisRackauckas-Claude]

Summary

• RootedTrees.jl already passes all ExplicitImports checks (no implicit imports, no stale explicit imports)
• Added ExplicitImports.jl tests to CI to maintain import hygiene going forward
• Added ExplicitImports to test dependencies in test/Project.toml
• Added ExplicitImports testset to test/runtests.jl

Changes

test/Project.toml:

• Added ExplicitImports = "7d51a73a-1435-4ff3-83d9-f097790105c7" to [deps]
• Added ExplicitImports = "1" to [compat]

test/runtests.jl:

• Added import: using ExplicitImports: check_no_implicit_imports, check_no_stale_explicit_imports
• Added testset:

@testset "ExplicitImports" begin
    @test isnothing(check_no_implicit_imports(RootedTrees))
    @test isnothing(check_no_stale_explicit_imports(RootedTrees))
end

Test plan

• Run Pkg.test() locally - all tests pass (580,749 tests passed)
• ExplicitImports checks pass

cc @ChrisRackauckas

🤖 Generated with Claude Code