Fix example notebooks for current release zips

[0x17]

• Bug fix: example notebooks failed on a fresh run because the release zip ships gamsconfig_cuopt.yaml, which the first cell never merged into gamsconfig.yaml (solve failed with unknown solver cuopt)
  • first cell now downloads fetch-cuoptlink.py from main and runs install -g <gamspy base> -c <cuversion> --cuda-runtime
  • skips the install if .cuopt_installed_files.txt already exists, so re-running the cell does not re-download ~1.5-2 GB
  • only the final state of each download progress line is kept in the cell output
• Bug fix: _test_installation in fetch-cuoptlink.py needed gamslib, which gamspy_base does not ship
  • now writes a small embedded LP model instead and aborts unless the model status is Optimal
  • README note updated accordingly
• Re-ran both notebooks (cu12, cu13) against v0.0.8e: GAMSPy 1.28.1 / GAMS 54.5.0, cuOpt 26.08.00, objective 153.697
• Verified: install + self-test on a GAMSPy base for cu12 and cu13; self-test fails (rc 3) for an infeasible variant of the test model; arm64 not tested
• Note: notebooks fetch the script from main, so they only work once this is merged