Allow ODE solutions to carry traced return codes

[ChrisRackauckas-Claude]

Draft. Ignore this PR until reviewed by @ChrisRackauckas.

What changed and why

ODESolution.retcode was fixed to ReturnCode.T. An OrdinaryDiffEq solve staged inside Reactant.@jit must carry a traced return code through the device loop, so Reactant cannot reconstruct the solution with that concrete field type. This appends the return-code type to ODESolution's parameters and keeps the documented ODESolution{T,N} constructor unchanged. It also lets successful_retcode classify scalar carriers convertible to ReturnCode.T.

This is the SciMLBase prerequisite for the OrdinaryDiffEq Reactant solve work. The generic successful_retcode fallback intentionally matches the one in #1563; whichever PR merges second should drop the duplicate line during conflict resolution.

Verification

After rebasing onto current upstream/master:

$ GROUP=Core julia +1.12.4 --project=. --startup-file=no -e 'using Pkg; Pkg.test()'
Test Summary:      | Pass  Total   Time
Solution interface |   84     84  18.7s
Test Summary: | Pass  Total     Time
Remake        | 4605   4605  4m30.7s
Testing SciMLBase tests passed

$ JULIA_CONDAPKG_BACKEND=Null JULIA_PYTHONCALL_EXE=/usr/bin/python3 GROUP=QA julia +1.12.4 --project=. --startup-file=no -e 'using Pkg; Pkg.test()'
Test Summary: | Pass  Total     Time
QA            |  123    123  3m18.6s
Testing SciMLBase tests passed

$ julia +release --startup-file=no -m Runic --check --diff Project.toml src/retcodes.jl src/solutions/ode_solutions.jl test/solution_interface.jl
$ typos Project.toml src/retcodes.jl src/solutions/ode_solutions.jl test/solution_interface.jl

The default CondaPkg-backed QA run also failed on unchanged master because the machine's shared Pixi cache contains zero-byte Python standard-library files. A standalone Pixi reproducer produced the same init_fs_encoding failure; unchanged master passed all 123 QA assertions with a fresh cache. No test was skipped or silenced.

Reviewer judgment

Appending a type parameter is observable to downstream code that spells every ODESolution parameter explicitly. The documented constructor takes only {T,N} and repository tests cover it, but the 3.51.0 minor bump should be reconsidered if full structural parameterization is treated as supported public API.

Not verified

• Downstream package suites; the companion OrdinaryDiffEq draft PR will exercise the Reactant path against this branch.
• GPU-specific paths, which this change does not touch.

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