Make selftest resilient to missing optional community dependency

[Copilot]

py3plex selftest was too strict for optional capabilities: missing python-louvain could fail the full self-check even when core functionality was healthy. This PR makes selftest distinguish required failures from optional capability skips.

• Selftest optional dependency behavior

  • Tracks optional module availability during dependency probing.
  • Treats community detection as skippable when community (python-louvain) is not installed.
  • Keeps community check strict when the optional module is present.

• Selftest summary semantics

  • Separates required pass/fail accounting from skipped checks.
  • Updates summary output to report:
    • required pass ratio
    • skipped count (when present)
  • Returns success when required checks pass, even if optional checks are skipped.

• CLI test coverage

  • Adds targeted test to simulate missing community import and assert:
    • selftest exits successfully
    • skip is explicitly reported
    • skipped count appears in summary

if not optional_status.get("community"):
    print("   [-] Community detection skipped: python-louvain is not installed")
    community_status = None