Allow snapshot-id in assert-ref-snapshot-id requirement to serialize to null in json (#2343)

Closes #2342

# Rationale for this change

The OpenAPI spec specifies that `snapshot-id` is required [1], even if
it's `null`. The current code omits the field b/c `exclude_none=True` is
set for all models that extend `IcebergBaseModel`. Setting
`exclude=False` on the field doesn't override the behavior and thus the
model needs to control it's own serialization. This is the only model I
know of so far that has this "required null" problem so I held back from
making another class for this model to extend.

# Are these changes tested?

Yes

# Are there any user-facing changes?

Set `snapshot-id: null` when committing tables that don't have a current
snapshot-id.

[1]:
https://github.com/apache/iceberg/blob/main/open-api/rest-catalog-open-api.yaml#L3138-L3155

---------

Co-authored-by: Fokko Driesprong <fokko@apache.org>
Co-authored-by: Kevin Liu <kevinjqliu@users.noreply.github.com>

Author: 3 people

pyiceberg/table/update/__init__.py

@@ -21,7 +21,7 @@
 from abc import ABC, abstractmethod
 from datetime import datetime
 from functools import singledispatch
-from typing import TYPE_CHECKING, Annotated, Any, Dict, Generic, List, Literal, Optional, Tuple, TypeVar, Union, cast
+from typing import TYPE_CHECKING, Annotated, Any, Dict, Generic, List, Literal, Optional, Set, Tuple, TypeVar, Union, cast
 
 from pydantic import Field, field_validator, model_validator
 
@@ -745,6 +745,13 @@ def validate(self, base_metadata: Optional[TableMetadata]) -> None:
         elif self.snapshot_id is not None:
             raise CommitFailedException(f"Requirement failed: branch or tag {self.ref} is missing, expected {self.snapshot_id}")
 
+    # override the override method, allowing None to serialize to `null` instead of being omitted.
+    def model_dump_json(
+        self, exclude_none: bool = False, exclude: Optional[Set[str]] = None, by_alias: bool = True, **kwargs: Any
+    ) -> str:
+        # `snapshot-id` is required in json response, even if null
+        return super().model_dump_json(exclude_none=False)
+
 
 class AssertLastAssignedFieldId(ValidatableTableRequirement):
     """The table's last assigned column id must match the requirement's `last-assigned-field-id`."""

tests/table/test_init.py

@@ -1039,6 +1039,9 @@ def test_assert_ref_snapshot_id(table_v2: Table) -> None:
     ):
         AssertRefSnapshotId(ref="test", snapshot_id=3055729675574597004).validate(base_metadata)
 
+    expected_json = '{"type":"assert-ref-snapshot-id","ref":"main","snapshot-id":null}'
+    assert AssertRefSnapshotId(ref="main").model_dump_json() == expected_json
+
 
 def test_assert_last_assigned_field_id(table_v2: Table) -> None:
     base_metadata = table_v2.metadata