Use SciMLTesting API docs QA

[ChrisRackauckas-Claude]

Summary

Ignore until reviewed by @ChrisRackauckas.

Follow-up to #185, which was merged before the latest instruction to use SciMLTesting's built-in API-docs checker.

• Replace the hand-rolled public API docs @testset with run_qa(...; api_docs_kwargs = (; rendered = true)).
• Pin test/qa/Project.toml to SciMLTesting 2.1.0 from SciML/SciMLTesting.jl@a5cecca928f2c684c23505c3cd83921d71753e2e, since General currently resolves only 1.x.
• Activate the QA project before loading SciMLTesting when GROUP=QA, so the grouped-tests entrypoint loads the pinned SciMLTesting version instead of the root test dependency.
• Add SafeTestsets to the QA environment and Pkg to root test extras because the grouped test runner needs SafeTestsets after activating test/qa, and test/runtests.jl now imports Pkg during QA setup.

Validation

• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.12 -e 'using Pkg; Pkg.activate(; temp=true); Pkg.add(PackageSpec(name="Runic", version="1")); using Runic; ...' — Runic check passed for test/runtests.jl and test/qa/qa.jl.
• timeout 3600 env GROUP=QA /home/crackauc/.juliaup/bin/julia +1.12 --project=. -e 'using Pkg; Pkg.test(; coverage=true, julia_args=["--check-bounds=yes", "--compiled-modules=yes", "--depwarn=yes"], force_latest_compatible_version=false, allow_reresolve=true)' — CI-shaped QA Pkg.test passed with SciMLTesting 2.1.0 from the pinned commit.
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=test/qa -e 'using Pkg; Pkg.add(PackageSpec(url="https://github.com/SciML/SciMLTesting.jl", rev="a5cecca928f2c684c23505c3cd83921d71753e2e")); Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); using SciMLTesting; @assert isdefined(SciMLTesting, :run_api_docs); include("test/qa/qa.jl")' — lts QA passed against the local checkout and pinned SciMLTesting commit.