Stabilize Braindecode py3.12 CI by capping xdist parallelism

[Copilot]

The dev ubuntu-latest, py-3.12 Braindecode workflow was not failing on a test assertion; the runner was being shut down mid-suite while executing the test matrix with pytest-xdist auto-parallelism. This change makes the workflow deterministic on GitHub-hosted runners by reducing process fan-out.

• CI workflow

  • Limit Braindecode test parallelism in .github/workflows/test-braindecode.yml
  • Replace unbounded worker selection (-n auto) with a fixed worker count (-n 4)

• Why this change

  • -n auto scales to the full runner CPU count, which can overcommit memory for the Braindecode test suite on ubuntu-latest
  • A fixed worker count preserves parallel execution while avoiding runner shutdowns partway through the job

• Behavioral impact

  • No product code changes
  • No test selection changes
  • Only the CI execution strategy is adjusted

# .github/workflows/test-braindecode.yml
- pytest test/ --quiet -n auto
+ pytest test/ --quiet -n 4