fix: Set Expression serialization to use 'values' (#2782)

Related to  #2775 

# Rationale for this change

Update the Set Expression serialization to align with the REST spec.
Today, the set expression literals are serialized under items, instead
of values.

For instance:

```json

{
  "term": "foo",
  "type": "not-in",
  "items": [
    1,
    2,
    3
  ]
}

```
When it should be:
```json
{
  "term": "foo",
  "type": "not-in",
  "values": [
    1,
    2,
    3
  ]
}
```

**Rest ref**:
https://github.com/apache/iceberg/blob/47d5f5009eafbbb526e6e2c9cbeac3105bf34670/open-api/rest-catalog-open-api.yaml#L2353

**Expression ref**:
https://github.com/apache/iceberg/blob/d19e3ff07653167d902865281601a5da4e2f2def/core/src/main/java/org/apache/iceberg/expressions/ExpressionParser.java#L189-L192

## Are these changes tested?

Yes

## Are there any user-facing changes?

only serialization, and initialization stays the same.

Author: geruh

pyiceberg/expressions/__init__.py

@@ -596,11 +596,11 @@ class SetPredicate(IcebergBaseModel, UnboundPredicate, ABC):
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
     type: TypingLiteral["in", "not-in"] = Field(default="in")
-    literals: set[LiteralValue] = Field(alias="items")
+    literals: set[LiteralValue] = Field(alias="values")
 
     def __init__(self, term: str | UnboundTerm, literals: Iterable[Any] | Iterable[LiteralValue]):
         literal_set = _to_literal_set(literals)
-        super().__init__(term=_to_unbound_term(term), items=literal_set)  # type: ignore
+        super().__init__(term=_to_unbound_term(term), values=literal_set)  # type: ignore
         object.__setattr__(self, "literals", literal_set)
 
     def bind(self, schema: Schema, case_sensitive: bool = True) -> BoundSetPredicate:

tests/expressions/test_expressions.py

@@ -915,12 +915,12 @@ def test_not_in() -> None:
 
 def test_serialize_in() -> None:
     pred = In(term="foo", literals=[1, 2, 3])
-    assert pred.model_dump_json() == '{"term":"foo","type":"in","items":[1,2,3]}'
+    assert pred.model_dump_json() == '{"term":"foo","type":"in","values":[1,2,3]}'
 
 
 def test_serialize_not_in() -> None:
     pred = NotIn(term="foo", literals=[1, 2, 3])
-    assert pred.model_dump_json() == '{"term":"foo","type":"not-in","items":[1,2,3]}'
+    assert pred.model_dump_json() == '{"term":"foo","type":"not-in","values":[1,2,3]}'
 
 
 def test_bound_equal_to(term: BoundReference) -> None: