Handle missing parent org web settings without crashing import/apply

[Copilot]

Import/apply for some orgs (notably eclipse-cfm) fails when several web-only org settings are absent in GitHub UI and retrieval reaches has_discussions. The current flow can raise on dependent settings when their parent setting was not retrieved.

• Web settings retrieval hardening

  • Updated WebClient._retrieve_settings to guard parent-dependent settings lookup.
  • When a dependent setting declares parent:
    • if parent retrieval failed but was requested, skip the dependent setting (warn, continue),
    • if parent was not requested, set dependent value to None,
    • if parent exists and is False, keep existing behavior (None + continue).

• Targeted regression coverage

  • Added focused async tests for discussions settings retrieval:
    • parent retrieval failure no longer triggers crash path,
    • omitted parent key yields {"discussion_source_repository": None} for dependent setting.

Example of the new parent guard behavior:

parent = setting_def.get("parent")
if parent is not None:
    if parent not in settings:
        if parent not in included_keys:
            settings[setting] = None
            continue
        _logger.warning(
            "failed to retrieve parent setting '%s', skipping dependent setting '%s'",
            parent, setting
        )
        continue