Simplify root CI to grouped-tests.yml (matrix from test/test_groups.toml)

[ChrisRackauckas-Claude]

Summary

Replaces the hand-maintained group × version matrix in the root CI.yml test job with the thin grouped-tests.yml@v1 caller. The root package's test-group matrix is now read from test/test_groups.toml (already present from the merged monorepo uniformization).

Before — CI.yml embedded:

  test:
    name: "Tests"
    strategy:
      matrix:
        group: [Core, nopre]
        version: ['1', 'lts', 'pre']
        exclude:
          - group: nopre
            version: 'pre'
    uses: "SciML/.github/.github/workflows/tests.yml@v1"
    with:
      julia-version: "${{ matrix.version }}"
      group: "${{ matrix.group }}"
      coverage-directories: "src"

After:

  tests:
    uses: "SciML/.github/.github/workflows/grouped-tests.yml@v1"
    with:
      coverage-directories: "src"
    secrets: "inherit"

The matrix now lives in test/test_groups.toml:

[Core]
versions = ["lts", "1", "pre"]

[nopre]
versions = ["lts", "1"]

Verified matrix match — 5/5 exact

Ran scripts/compute_affected_sublibraries.jl . --root-matrix from SciML/.github@v1 and compared the emitted (group, version) set against the set the old workflow ran:

group	versions
Core	lts, 1, pre
nopre	lts, 1 (pre excluded)

No missing, no extra. 5/5 exact.

Notes

• name: CI, the on: triggers, and concurrency: are preserved verbatim, so the branch-protection status-check identity is unchanged.
• The root test/runtests.jl dispatches on the default GROUP env var, so no group-env-name input is needed.
• check-bounds and coverage defaults are unchanged (yes / true in both the old tests.yml@v1 path and grouped-tests.yml); only the non-default coverage-directories: "src" is carried over.
• The nopre group key is intentionally lowercase to match the literal GROUP == "nopre" dispatch in runtests.jl.
• SublibraryCI.yml (the lib/* sublibrary coverage via sublibrary-project-tests.yml@v1) is untouched.

Ignore until reviewed by @ChrisRackauckas.