`check_name_does_not_collide_with_schema` incorrectly rejects Void transform partitions whose names collide with schema columns

### Apache Iceberg Rust version

0.6.0 (latest version)

### Describe the bug

When a partition field's transform is changed from `Identity` to `Void` (to effectively drop the field in v1 tables), re-binding the unbound partition spec to the schema fails because
`check_name_does_not_collide_with_schema` treats `Void` the same as other non-Identity transforms and rejects the name collision.

However, `Void` transforms always produce `null` results, so they pose no semantic conflict with schema columns — just like `Identity` transforms sourced from the same column. The check should allow Void
transforms under the same condition as Identity.

### To Reproduce

```rust
use iceberg::spec::{
NestedField, PartitionSpec, Schema, Transform, Type, UnboundPartitionField, PrimitiveType,
};

let schema = Schema::builder()
.with_fields(vec![
NestedField::required(1, "id", Type::Primitive(PrimitiveType::Int)).into(),
])
.build()
.unwrap();

// Simulate a previously valid identity partition whose transform was changed to Void
let unbound = UnboundPartitionField {
source_id: 1,
field_id: Some(1000),
name: "id".to_string(), // same name as schema column
transform: Transform::Void,
};

// Binding back fails
let err = PartitionSpec::builder(schema)
.add_unbound_field(unbound)
.unwrap_err();

assert!(err.message().contains("conflicts with schema")); // currently passes (fails as expected)
// But it should succeed — Void transform produces only null, no semantic conflict
```

### Expected behavior

`check_name_does_not_collide_with_schema` should allow Void transforms when the partition name collides with a schema column and the source_id matches, treating Void the same as Identity for this check.

### Willingness to contribute

I can contribute a fix for this bug independently

Start at check_name_does_not_collide_with_schema and follow the PartitionSpec builder path that binds an UnboundPartitionField. Use the provided reproduction with a Void transform whose name matches the schema column, then verify binding succeeds when source_id matches and that the existing collision behavior remains unchanged for other transforms.

Written by the indexing model from the issue text.