fix: add portals.json validation tests (closes #1440)

The current portals.json on main is valid JSON. The malformed version
referenced in the issue was likely from a draft that was fixed before
merge. Added 4 validation tests to prevent future corruption:

- test_portals_json_valid: file must parse as JSON
- test_portals_json_no_duplicate_keys: no duplicate keys in any object
- test_portals_json_structure: required fields (id, name, description,
  status, color, position) present on every portal
- test_portals_json_positions_valid: x/y/z coordinates are numbers

Closes #1440

tests/test_portals_json.py

@@ -0,0 +1,51 @@
+"""Test portals.json integrity — valid JSON, no duplicate keys, expected structure."""
+
+from pathlib import Path
+import json
+
+
+def test_portals_json_valid():
+    """portals.json must be valid JSON."""
+    path = Path(__file__).resolve().parents[1] / "portals.json"
+    data = json.loads(path.read_text(encoding="utf-8"))
+    assert isinstance(data, list), "portals.json should be a JSON array"
+
+
+def test_portals_json_no_duplicate_keys():
+    """portals.json must not contain duplicate keys in any object."""
+    path = Path(__file__).resolve().parents[1] / "portals.json"
+    content = path.read_text(encoding="utf-8")
+
+    def check_duplicates(pairs):
+        keys = [k for k, _ in pairs]
+        seen = set()
+        for k in keys:
+            assert k not in seen, f"Duplicate key '{k}' found in portals.json"
+            seen.add(k)
+        return dict(pairs)
+
+    json.loads(content, object_pairs_hook=check_duplicates)
+
+
+def test_portals_json_structure():
+    """Each portal entry must have required fields."""
+    path = Path(__file__).resolve().parents[1] / "portals.json"
+    data = json.loads(path.read_text(encoding="utf-8"))
+
+    required = {"id", "name", "description", "status", "color", "position"}
+    for i, portal in enumerate(data):
+        assert isinstance(portal, dict), f"Portal [{i}] is not a dict"
+        missing = required - set(portal.keys())
+        assert not missing, f"Portal [{i}] ({portal.get('id', '?')}) missing fields: {missing}"
+
+
+def test_portals_json_positions_valid():
+    """Each portal position must have x, y, z coordinates."""
+    path = Path(__file__).resolve().parents[1] / "portals.json"
+    data = json.loads(path.read_text(encoding="utf-8"))
+
+    for i, portal in enumerate(data):
+        pos = portal.get("position", {})
+        for axis in ("x", "y", "z"):
+            assert axis in pos, f"Portal [{i}] ({portal.get('id')}) missing position.{axis}"
+            assert isinstance(pos[axis], (int, float)), f"Portal [{i}] position.{axis} is not a number"