Core: declare SparseArrays in [extras] so the test target survives weakdep promotion

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

The failure

Downgrade Sublibraries / test (lib/OrdinaryDiffEqCore) aborts before a single
test runs:

`SparseArrays` declared as a `test` dependency, but no such entry in `extras` or `weakdeps`

Why

SparseArrays is in lib/OrdinaryDiffEqCore's test target but was listed
only under [weakdeps]. That resolves fine normally — Pkg looks the test
target up in [extras] and [weakdeps]. But julia-downgrade-compat promotes
a weak dependency to a real one when another package in the merged project
needs it, and in this job it does exactly that:

[ Info: Promoting root weak dependency required by OrdinaryDiffEqNonlinearSolve: SparseArrays

Promotion moves the entry out of [weakdeps] and into [deps]. The
test target is then pointing at a name that is in neither [extras] nor
[weakdeps], so Pkg.test errors out during test-environment generation.

This is the only Project.toml in the repo with a test-target name that lives in
[weakdeps] but not [extras]:

julia> for f in vcat(["Project.toml"], [joinpath("lib",d,"Project.toml") for d in readdir("lib")])
           d = TOML.parsefile(f)
           tgt = Set(get(get(d,"targets",Dict()),"test",String[]))
           extras = Set(keys(get(d,"extras",Dict()))); weak = Set(keys(get(d,"weakdeps",Dict())))
           w = setdiff(intersect(tgt, weak), extras)
           isempty(w) || println(f, ": ", sort(collect(w)))
       end
lib/OrdinaryDiffEqCore/Project.toml: ["SparseArrays"]

The fix

Also list SparseArrays in [extras]. [weakdeps] and [extensions] are
untouched, so OrdinaryDiffEqCoreSparseArraysExt still loads conditionally —
the entry only gives the test target a UUID that survives promotion.

Verification

1. Reproduced the CI failure locally by replaying the promotion step
(SparseArrays: [weakdeps] → [deps]) on unmodified master:

$ julia +1.11 --project=lib/OrdinaryDiffEqCore -e 'using Pkg; Pkg.test()'
ERROR: `SparseArrays` declared as a `test` dependency, but no such entry in `extras` or `weakdeps`

2. Same promotion, with this patch — the abort is gone and the test env builds:

      Status `~/claude_tmp/jl_Zb5Z9v/Project.toml`
Precompiling project for configuration --code-coverage=none ...
     Testing Running tests...

3. Normal (un-promoted) path still green — full Core group on Julia 1.11,
matching the CI job's ODEDIFFEQ_TEST_GROUP=Core:

$ ODEDIFFEQ_TEST_GROUP=Core julia +1.11 --project=lib/OrdinaryDiffEqCore -e 'using Pkg; Pkg.test()'
...
   2044.3 ms  ✓ OrdinaryDiffEqCore → OrdinaryDiffEqCoreSparseArraysExt
...
     Testing OrdinaryDiffEqCore tests passed

The extension still precompiles, confirming the [weakdeps] role is intact.

4. Both spellings resolve on every supported Julia — a minimal package with
SparseArrays in [weakdeps]+[extras] (normal) and in [deps]+[extras]
(post-promotion) passes Pkg.test on 1.10, 1.11 and 1.12.

Relation to other PRs

#4012 includes this same one-line change bundled with two unrelated fixes; this
PR isolates it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TPHRh64BLfoXSzQ3AxKNwC