fix: refuse graph schemas a definition cannot carry, and take any Table.add input on insert

A node or edge type whose schema would read back as another type from its JSON form (timestamp units and time zones, durations, times, dictionaries) is refused before a request carries it. Inserting into a property graph registers DataFrame converters first, as Table.add does, and the async graph handles are in the API reference.

python/src/graph.rs

@@ -29,10 +29,11 @@ impl GraphKind {
     }
 }
 
-/// A pyarrow schema as the JSON Arrow schema of a node or edge type.
+/// A pyarrow schema as the JSON Arrow schema of a node or edge type, refused
+/// when that form would not read back as the same schema.
 #[pyfunction]
 pub fn graph_schema_to_json(schema: PyArrowType<Schema>) -> PyResult<String> {
-    let json = lance_namespace::schema::arrow_schema_to_json(&schema.0)
+    let json = lancedb::graph::schema_to_json(&schema.0)
         .map_err(|error| PyValueError::new_err(error.to_string()))?;
     serde_json::to_string(&json).map_err(|error| PyValueError::new_err(error.to_string()))
 }