test: add CLI coverage tests to maintain >90% code coverage

Add 18 new tests covering text/JSON output for: help/usage, parse errors,
subnet, supernet, wide supernet, contain, overlap, subtract, list IP,
multiple networks, default CIDR, and examples in help.

Line coverage: 89.29% → 94.05%
CLI module coverage: 75.51% → 91.56%

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lduchosal

.config/dotnet-tools.json

@@ -6,13 +6,29 @@
       "version": "4.0.1",
       "commands": [
         "csmacnz.Coveralls"
-      ]
+      ],
+      "rollForward": false
     },
     "versionize": {
       "version": "2.5.0",
       "commands": [
         "versionize"
-      ]
+      ],
+      "rollForward": false
+    },
+    "coverlet.console": {
+      "version": "8.0.1",
+      "commands": [
+        "coverlet"
+      ],
+      "rollForward": false
+    },
+    "dotnet-coverage": {
+      "version": "18.5.2",
+      "commands": [
+        "dotnet-coverage"
+      ],
+      "rollForward": false
     }
   }
 }