Fix QA/Core CI: drop unmatched sublibrary [sources] from test/qa/Project.toml

[ChrisRackauckas-Claude]

Problem

Master CI is red. The tests / Core (julia 1), tests / Core (julia pre), and tests / QA (julia 1) jobs all fail at environment activation:

ERROR: LoadError: Sources for `JumpProblemLibrary` not listed in `deps` or `extras` section
at ".../test/qa/Project.toml".

test/qa/Project.toml listed every sublibrary (BVProblemLibrary, DAEProblemLibrary, DDEProblemLibrary, JumpProblemLibrary, NonlinearProblemLibrary, ODEProblemLibrary, SDEProblemLibrary) under [sources], but only DiffEqProblemLibrary is declared in [deps]. On the current CI Julia (1.12.6), Pkg.activate validates the project and rejects any [sources] key without a matching [deps]/[extras] entry.

This breaks both the QA group and the Core group, since Core reaches the same activate_qa_env path through the Core => QA umbrella in runtests.jl.

Fix

The qa env only needs DiffEqProblemLibrary (the QA test, qa/qa.jl, just runs ExplicitImports checks on DiffEqProblemLibrary). The root package's own [sources] transitively resolve the sublibraries, so the per-sublibrary [sources] entries in the qa env were spurious. Keep only DiffEqProblemLibrary = {path = "../.."}.

Local verification (Julia 1.12.6, same as CI's "julia 1")

Before (reproduces the CI failure):

ACTIVATE_ERROR: Sources for `JumpProblemLibrary` not listed in `deps` or `extras` section ...

After (this branch):

ACTIVATE_OK
INSTANTIATE_OK
Test Summary:   | Pass  Total   Time
ExplicitImports |    2      2  15.8s
QA_GROUP_OK

This does not address the separate downgrade-sublibraries / test (lib/BVProblemLibrary) failure (UndefVarError: BVPFunction not defined at the downgrade compat floor), which is a pre-existing downgrade compat-floor issue tracked separately.

Please ignore until reviewed by @ChrisRackauckas.