Fix #2377: Reorganize the test helpers (where appropriate) to cuda_python_test_helpers - #2384

• Confirmed: this is unrelated to PR 2509 and is deterministic, not a flake.

  - The job passes all 1,276 Pathfinder tests, then fails on the first bindings import with ModuleNotFoundError: No module named 'cuda'.
  - Pixi never builds cuda-bindings, so PR 2509’s modified build hooks are never executed.
  - The same failure already occurred in PR 2384, which was merged as 19e66499c2.
  - PR 2384 changed both pixi.toml files without updating their lockfiles. With Pixi 0.73, the first automatic lock refresh produces an
    environment missing the local cuda-bindings and cuda-pathfinder packages.

  - I reproduced the exact failure from a clean checkout using the CI’s Pixi 0.73. A second invocation in the same checkout converges and
    succeeds, but rerunning GitHub Actions starts fresh and repeats the first-pass failure.

  - The new TOML table placement also appears to unintentionally move several existing test dependencies from Conda to PyPI.

  I recommend fixing this separately from PR 2509:

  1. Keep the existing test dependencies under [feature.test.dependencies].
  2. Put only cuda-python-test-helpers under [feature.test.pypi-dependencies], assuming that dependency is intentional.
  3. Regenerate both cuda_bindings/pixi.lock and cuda_core/pixi.lock with Pixi 0.73 until they are stable and pixi lock --check passes from
     a clean checkout.

  4. Exercise both source-build/import commands.

  The same stale-lock problem exists in cuda_core, so fixing only the bindings lock would likely expose the core failure next. Optional
  hardening would be to make CI use locked mode so future manifest/lock mismatches fail explicitly instead of silently rewriting the lock.