fix(api): align zone content kind validation with model

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

tests/test_zone_content_kind.py

@@ -1,4 +1,4 @@
-"""Zone content_kind is fixed after create."""
+"""Zones may hold both presets and sequences."""
 
 import json
 import os
@@ -12,7 +12,7 @@ sys.path.insert(0, str(PROJECT_ROOT / "src"))
 from models.zone import Zone  # noqa: E402
 
 
-def test_update_cannot_change_content_kind():
+def test_zone_presets_and_sequences_can_coexist():
     with tempfile.TemporaryDirectory() as tmp:
         path = os.path.join(tmp, "zone.json")
         with open(path, "w", encoding="utf-8") as f:
@@ -20,8 +20,16 @@ def test_update_cannot_change_content_kind():
         z = Zone()
         z.file = path
         z.clear()
-        zid = z.create("preset zone", group_ids=[], content_kind="presets")
-        z.update(zid, {"content_kind": "sequences", "name": "preset zone"})
+        zid = z.create("mixed zone", group_ids=[], content_kind="presets")
+        z.update(
+            zid,
+            {
+                "presets": [["p1", "p2"]],
+                "sequence_ids": ["seq1"],
+            },
+        )
         doc = z.read(zid)
-        assert doc["content_kind"] == "presets"
-        assert doc.get("sequence_ids") == []
+        assert doc.get("sequence_ids") == ["seq1"]
+        preset_ids = Zone._preset_ids_in_doc(doc)
+        assert "p1" in preset_ids
+        assert "p2" in preset_ids