Improve explicit imports hygiene

[ChrisRackauckas-Claude]

Summary

This PR addresses all explicit imports issues identified by ExplicitImports.jl and adds CI tests to prevent regressions.

Changes Made

1. Fixed implicit imports in src/ODEInterfaceDiffEq.jl:

   • Added explicit imports for BinaryMaxHeap, BinaryMinHeap, counter from DataStructures
   • Added explicit imports for CallbackSet, ReturnCode, VectorContinuousCallback, check_keywords, warn_compat from SciMLBase
   • Added explicit import for I from LinearAlgebra
   • Added explicit module imports for Compat, FunctionWrappers, ODEInterface, SciMLBase
   • Added explicit import for module name DiffEqBase to support qualified function extensions
   • Added initialize! and savevalues! to DiffEqBase imports for proper function extension

2. Added ExplicitImports test suite:

   • Created test/explicit_imports.jl to check for implicit and stale imports
   • Added ExplicitImports to [extras] section in Project.toml
   • Integrated test into runtests.jl

3. All tests pass: Full test suite runs successfully including the new ExplicitImports checks

Benefits

• Improves code maintainability by making all dependencies explicit
• Prevents accidental reliance on transitive dependencies
• Adds CI checks to catch future import hygiene issues
• Follows Julia package best practices

Test Results

Test Summary:    | Pass  Total  Time
Explicit Imports |    2      2  1.5s

All existing tests continue to pass.

cc @ChrisRackauckas

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