fix(python): validate nested input (vertices, edges) instead of defaulting silently

The generated dict-to-struct conversions for nested records defaulted every
absent field and ignored unknown keys, so an LwPolyline built from
{'x': .., 'y': ..} vertices became zero-coordinate vertices. They now refuse an
unknown key (naming the keys the record takes) and a manifest list,
required_struct_fields, enforces the fields that define a nested record's
geometry (vertex locations, hatch edge ends and centres, section symbol points).
Regression test over real IPC; every existing lifecycle audit still passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/opencad-python/entity_manifest.json

@@ -973,5 +973,46 @@
         }
       }
     }
+  },
+  "required_struct_fields": {
+    "LwVertex": [
+      "location"
+    ],
+    "Vertex2D": [
+      "location"
+    ],
+    "Vertex3D": [
+      "location"
+    ],
+    "Vertex3DPolyline": [
+      "position"
+    ],
+    "PolygonMeshVertex": [
+      "location"
+    ],
+    "PolyfaceVertex": [
+      "location"
+    ],
+    "MLineVertex": [
+      "position"
+    ],
+    "LineEdge": [
+      "start",
+      "end"
+    ],
+    "CircularArcEdge": [
+      "center",
+      "radius"
+    ],
+    "EllipticArcEdge": [
+      "center",
+      "major_axis_endpoint"
+    ],
+    "PolylineEdge": [
+      "vertices"
+    ],
+    "SectionSymbolPoint": [
+      "point"
+    ]
   }
 }