Document and cap self-energy worker CPU usage

[Copilot]

The review comment requested documenting the newly added n_cpus argument for compute_all_self_energy and clarifying how it interacts with n_jobs and memory-based limiting.

• CPU cap behavior
  • _get_safe_n_jobs accepts an optional n_cpus cap and applies it before memory-based limiting.
• Public API docs
  • compute_all_self_energy now documents n_cpus and its interaction with n_jobs and memory limits.
• Test coverage
  • Added a targeted unit test validating that n_cpus caps auto-detected worker counts.

safe_n_jobs = _get_safe_n_jobs(
    lead_L,
    lead_R,
    requested_n_jobs=n_jobs,
    kpoint=sample_kpoint,
    n_cpus=n_cpus,
)