CaseWhen does not work with custom implemented column expression

### Describe the bug

While upgrading to DataFusion 52 (I know, I'm getting to 53 slowly) tests started failing for CASE WHEN for us because we have cases where we use custom `Column` `PhysicalExpr` rather than DataFusion one.

this should not affect correctness.

This was done as an optimization in this PR by @pepijnve
- #18329

because it find the columns referenced by traversing the expression tree and finding the `Column` Physical Expression. however, when having custom `Column` impl it will not add that used column to the projection

### To Reproduce

This is the same test as:
https://github.com/apache/datafusion/blob/1a0af76f780708ff9d9e1940686748bd743352d3/datafusion/physical-expr/src/expressions/case.rs#L1694-L1732

but using Custom impl of `Column`
```rust
#[test]
fn case_without_expr_and_with_custom_column_impl() -> Result<()> {

/// Custom impl of column
#[derive(Debug, Hash, PartialEq, Eq, Clone)]
pub struct CustomColumn {
/// The name of the column (used for debugging and display purposes)
name: String,
/// The index of the column in its schema
index: usize,
data_type: DataType,
nullable: bool,
}

impl CustomColumn {
pub fn new_with_schema(name: &str, schema: &Schema) -> Result> {
let index = schema.index_of(name)?;
let field = schema.field(index);
Ok(Arc::new(CustomColumn {
name: name.to_string(),
index,
data_type: field.data_type().clone(),
nullable: field.is_nullable(),
}))
}
}

impl std::fmt::Display for CustomColumn {
fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
write!(f, "{}@{}", self.name, self.index)
}
}

impl PhysicalExpr for CustomColumn {
fn as_any(&self) -> &dyn Any {
self
}

fn data_type(&self, _input_schema: &Schema) -> Result {
Ok(self.data_type.clone())
}

fn nullable(&self, _input_schema: &Schema) -> Result {
Ok(self.nullable)
}

fn evaluate(&self, batch: &RecordBatch) -> Result {
self.bounds_check(batch.schema().as_ref())?;
Ok(ColumnarValue::Array(Arc::clone(batch.column(self.index))))
}

fn children(&self) -> Vec<&Arc> {
vec![]
}

fn with_new_children(
self: Arc,
_children: Vec>,
) -> Result> {
Ok(self)
}

fn fmt_sql(&self, _: &mut Formatter<'_>) -> std::fmt::Result {
unimplemented!()
}
}

impl CustomColumn {
fn bounds_check(&self, input_schema: &Schema) -> Result<()> {
if self.index < input_schema.fields.len() {
Ok(())
} else {
internal_err!(
"PhysicalExpr CustomColumn references column '{}' at index {} (zero-based) but input schema only has {} columns: {:?}",
self.name,
self.index,
input_schema.fields.len(),
input_schema.fields().iter().map(|f| f.name()).collect::>()
)
}
}
}

let batch = case_test_batch()?;
let schema = batch.schema();

// CASE WHEN a = 'foo' THEN 123 WHEN a = 'bar' THEN 456 END
let when1 = binary(
CustomColumn::new_with_schema("a", &schema)?,
Operator::Eq,
lit("foo"),
&batch.schema(),
)?;
let then1 = lit(123i32);
let when2 = binary(
CustomColumn::new_with_schema("a", &schema)?,
Operator::Eq,
lit("bar"),
&batch.schema(),
)?;
let then2 = lit(456i32);

let expr = generate_case_when_with_type_coercion(
None,
vec![(when1, then1), (when2, then2)],
None,
schema.as_ref(),
)?;
let result = expr
.evaluate(&batch)?
.into_array(batch.num_rows())
.expect("Failed to convert to array");
let result = as_int32_array(&result)?;

let expected = &Int32Array::from(vec![Some(123), None, None, Some(456)]);

assert_eq!(expected, result);

Ok(())
}
```

### Expected behavior

to work with any custom column impl, and hopefully still having that optimization

### Additional context

also might affect lambda support:
- https://github.com/apache/datafusion/pull/18921#discussion_r3006009469

Start with the linked test in datafusion/physical-expr/src/expressions/case.rs and compare it with the supplied custom Column reproduction. Read PR #18329 to understand the projection optimization, and review the lambda-support discussion in PR #18921. Done means the custom-column CASE WHEN test passes without losing the optimization for supported expressions.

Written by the indexing model from the issue text.