fix: address network command review feedback

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/bsk-protocol/schema/tool_network_entry.json

@@ -4,8 +4,7 @@
   "type": "object",
   "required": [
     "kind",
-    "sequence",
-    "url"
+    "sequence"
   ],
   "properties": {
     "error_text": {
@@ -68,7 +67,10 @@
       "type": "boolean"
     },
     "url": {
-      "type": "string"
+      "type": [
+        "string",
+        "null"
+      ]
     }
   },
   "definitions": {