Improve explicit imports hygiene

[ChrisRackauckas-Claude]

Summary

This PR improves explicit imports hygiene for the GeometricIntegratorsDiffEq.jl package using ExplicitImports.jl.

Changes made:

1. Made all imports explicit in src/GeometricIntegratorsDiffEq.jl:

   • Explicitly imported Reexport and @reexport from Reexport.jl
   • Made DiffEqBase import explicit in the @reexport statement
   • Explicitly imported functions from SciMLBase: ReturnCode, check_keywords, isinplace, warn_compat
   • Explicitly imported all needed integrator types and functions from GeometricIntegrators: CrankNicolson, Crouzeix, ExplicitEuler, ExplicitMidpoint, Gauss, Heun2, Heun3, ImplicitEuler, ImplicitMidpoint, KraaijevangerSpijker, Kutta3, LobattoIIIA, LobattoIIIAIIIB, LobattoIIIB, LobattoIIIBIIIA, LobattoIIIC, LobattoIIID, LobattoIIIE, LobattoIIIF, Newton, QinZhang, RK4, RK416, RK438, RadauIA, RadauIIA, Ralston2, Ralston3, Runge2, SRK3, SSPRK3, SymplecticEulerA, SymplecticEulerB, integrate

2. Fixed test file imports in test/runtests.jl:

   • Added explicit imports for solve, ReturnCode, and SecondOrderODEProblem from DiffEqBase

3. Added ExplicitImports.jl tests to prevent regression:

   • Created test/explicit_imports_test.jl with tests for implicit and stale imports
   • Added ExplicitImports to test dependencies in Project.toml
   • Added ExplicitImports test group to test/runtests.jl

Benefits:

• No reliance on transitive dependencies: All imports are now explicit, reducing risk of breakage from upstream changes
• Better code clarity: It's immediately clear which functions come from which packages
• Automated testing: ExplicitImports tests will catch any future implicit imports before they reach CI

Testing:

All tests pass, including:

• ✅ Standard ODE Problems tests (32 tests)
• ✅ ExplicitImports tests (2 tests)
• ✅ Allocation tests (9 tests)
• ✅ JET static analysis tests (29 tests)

cc @ChrisRackauckas

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