fix: Serialize AlwaysTrue/AlwaysFalse as boolean literals (#2780)

related to #2775 

# Rationale for this change

This PR aligns the `AlwaysTrue` and `AlwaysFalse` expression
serialization to use the boolean primitives `true`/`false` instead of
string representation, matching the Iceberg
[ExpressionParser](https://github.com/apache/iceberg/blob/main/core/src/main/java/org/apache/iceberg/expressions/ExpressionParser.java#L101-L108).

I know the spec defines the true/false expressions as string but today
we can't hit the scan planning client endpoint following these models.

I know the spec defines the true/false expressions as a string
representation, but currently we can't successfully use the scan
planning client endpoint with those models. Java's actual implementation
serializes these as boolean literals.

For instance, with what we have today we would throw an illegal argument
exception:

```
{
    "snapshot-id": 2540284336700708540,
    "filter": "true",
    "case-sensitive": true
}
```
Throws: `java.lang.IllegalArgumentException: argument
"content" is null`

But, this works just fine (notice no quotes):

```
{
    "snapshot-id": 2540284336700708540,
    "filter": true,
    "case-sensitive": true
}

```

## Are these changes tested?

Yes

## Are there any user-facing changes?

No, this only affects the JSON serialization format to match Java Cores
behavior.

Author: geruh

pyiceberg/expressions/__init__.py

@@ -370,10 +370,10 @@ def __getnewargs__(self) -> tuple[BooleanExpression]:
         return (self.child,)
 
 
-class AlwaysTrue(BooleanExpression, Singleton, IcebergRootModel[str]):
+class AlwaysTrue(BooleanExpression, Singleton, IcebergRootModel[bool]):
     """TRUE expression."""
 
-    root: str = "true"
+    root: bool = True
 
     def __invert__(self) -> AlwaysFalse:
         """Transform the Expression into its negated version."""
@@ -388,10 +388,10 @@ def __repr__(self) -> str:
         return "AlwaysTrue()"
 
 
-class AlwaysFalse(BooleanExpression, Singleton, IcebergRootModel[str]):
+class AlwaysFalse(BooleanExpression, Singleton, IcebergRootModel[bool]):
     """FALSE expression."""
 
-    root: str = "false"
+    root: bool = False
 
     def __invert__(self) -> AlwaysTrue:
         """Transform the Expression into its negated version."""

tests/expressions/test_expressions.py

@@ -770,7 +770,7 @@ def test_not_json_serialization_and_deserialization() -> None:
 
 def test_always_true() -> None:
     always_true = AlwaysTrue()
-    assert always_true.model_dump_json() == '"true"'
+    assert always_true.model_dump_json() == "true"
     assert str(always_true) == "AlwaysTrue()"
     assert repr(always_true) == "AlwaysTrue()"
     assert always_true == eval(repr(always_true))
@@ -779,7 +779,7 @@ def test_always_true() -> None:
 
 def test_always_false() -> None:
     always_false = AlwaysFalse()
-    assert always_false.model_dump_json() == '"false"'
+    assert always_false.model_dump_json() == "false"
     assert str(always_false) == "AlwaysFalse()"
     assert repr(always_false) == "AlwaysFalse()"
     assert always_false == eval(repr(always_false))