Interface compatibility: Fix numeric type preservation for BigFloat/Float32

[ChrisRackauckas-Claude]

Summary

This PR improves numeric type interface compliance in DiffEqFinancial.jl, ensuring proper type preservation when using BigFloat or Float32 instead of the default Float64.

Changes Made

1. MfStateProblem (src/problems.jl:143): Changed return 0 to return zero(u) to preserve the numeric type
2. BlackScholesProblem (src/problems.jl:87): Changed (t) -> 0 to (t) -> zero(T) for the dividend yield function
3. HestonProblem (src/problems.jl:25-34): Properly propagate eltype(u0) to:
   • Covariance matrix Γ
   • Noise process vectors (zeros(T, 2) instead of zeros(2))

Interface Testing Performed

• BigFloat support: All scalar problems (GeometricBrownianMotion, OrnsteinUhlenbeck, ExtendedOrnsteinUhlenbeck, CIR, MfState, BlackScholes) now work correctly with BigFloat
• Float32 support: HestonProblem and other problems now correctly preserve Float32 element types
• JLArray (GPU) testing: HestonProblem fundamentally requires scalar indexing for its 2D coupled SDE system, so GPU array support would require significant algorithm redesign (not addressed in this PR)

Tests Added

Added test/interface_tests.jl with comprehensive tests for:

• BigFloat support for all scalar financial SDE problems
• Float32 support for HestonProblem and GeometricBrownianMotion

All existing tests continue to pass.

Test plan

• Run Pkg.test() locally - all tests pass
• Verify BigFloat type preservation in solution output
• Verify Float32 type preservation in HestonProblem

cc @ChrisRackauckas

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🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Changes made to separate JET tests into nopre group

Following the OrdinaryDiffEq.jl pattern, I've restructured the tests:

New Directory Structure

test/
├── explicit_imports.jl
├── interface_tests.jl
├── runtests.jl           # GROUP-based test dispatcher
└── jet/
    ├── Project.toml      # Separate deps: JET, AllocCheck
    ├── runtests.jl       # Runs jet_tests.jl and alloc_tests.jl
    ├── jet_tests.jl
    └── alloc_tests.jl

Key Changes

1. test/jet/Project.toml: Separate environment with JET and AllocCheck dependencies
2. test/runtests.jl: Uses GROUP env variable to control which tests run
   • GROUP=Core (default): Runs explicit imports, interface tests, main tests
   • GROUP=JET: Activates jet environment and runs JET/AllocCheck tests
   • GROUP=All: Runs everything
3. Project.toml: Removed JET and AllocCheck from main test dependencies
4. CI workflow:
   • Core tests run on Julia 1, lts, and pre
   • JET tests only run on Julia 1 and lts (excluded from pre to avoid precompilation issues)

This follows the same pattern used by OrdinaryDiffEq.jl for separating tests that don't work well on prerelease Julia versions.