fix: preserve write_default when applying a name mapping

NameMappingProjectionVisitor.field rebuilt each NestedField with an
initial_write= keyword, but NestedField.__init__ has no such parameter
(the write-default keyword is write_default). Being a Pydantic model,
the unknown key was swallowed by **data and silently ignored, so
apply_name_mapping dropped every field's write_default to None.

Pass write_default= to match the NestedField constructor and add a
regression test asserting both initial_default and write_default
survive a name mapping.

Author: anxkhn

pyiceberg/table/name_mapping.py

@@ -341,7 +341,7 @@ def field(self, field: NestedField, field_partner: MappedField | None, field_res
             required=field.required,
             doc=field.doc,
             initial_default=field.initial_default,
-            initial_write=field.write_default,
+            write_default=field.write_default,
         )
 
     def list(self, list_type: ListType, list_partner: MappedField | None, element_result: IcebergType) -> IcebergType:

tests/table/test_name_mapping.py

@@ -403,3 +403,22 @@ def test_mapping_using_by_visitor(table_schema_nested: Schema, table_name_mappin
         ),
     )
     assert apply_name_mapping(schema_without_ids, table_name_mapping_nested).fields == table_schema_nested.fields
+
+
+def test_mapping_preserves_field_defaults() -> None:
+    schema_without_ids = Schema(
+        NestedField(
+            field_id=0,
+            name="foo",
+            field_type=StringType(),
+            required=False,
+            initial_default="init",
+            write_default="write",
+        ),
+    )
+    name_mapping = NameMapping([MappedField(field_id=1, names=["foo"])])
+
+    mapped_field = apply_name_mapping(schema_without_ids, name_mapping).fields[0]
+
+    assert mapped_field.initial_default == "init"
+    assert mapped_field.write_default == "write"