overhaul CI: simplify to Python 3.12 + PyTorch 2.6, fix dependency versions

- Simplify CI from 16 jobs to 4: unit tests, integration tests (6 groups),
  distributed tests, and minimal install
- Remove hardcoded ray==2.9.0 (doesn't exist for Python 3.12); let pip
  resolve ray>=2.9 from requirements_distributed.txt
- Remove Python 3.10/3.11 matrix (only test on 3.12)
- Remove LLM test job and combinatorial test job (separate concerns)
- Remove torchtext/sed hacks for requirements stripping
- Remove macOS conditional steps (ubuntu-only CI)
- Update ConfigSpace==0.7.1 → >=1.0 (0.7.1 has no py3.12 binary wheels)
- Remove deepspeed from requirements_distributed.txt (needs CUDA to build,
  GPU-only feature; already skipped in CPU-only CI)
- Remove getdaft pins (unused in Ludwig codebase)
- Remove horovod from requirements_extra.txt
- Remove sqlalchemy<2 pin (aim 3.29.1 supports sqlalchemy 2.x)
- Add pip caching and artifact uploads to all test jobs

Author: w4nderlust