Improve IcebergCommitExec to correctly populate properties/schema (#1721)

## Which issue does this PR close?

PR fixes schema mismatch errors (similar to the example shown below)
when using `IcebergCommitExec` with DataFusion. This occurs when
`IcebergCommitExec` is not the top-level plan but is instead wrapped as
the input to another plan node, for example when added by a custom
optimization rule (cache invalidation step for example).

>An internal error occurred. Internal error: PhysicalOptimizer rule
'OutputRequirements' failed. Schema mismatch. Expected original schema:
Schema { fields: [Field { name: "count", data_type: UInt64, nullable:
false, dict_id: 0, dict_is_ordered: false, metadata: {} }], metadata: {}
}, got new schema: Schema { fields: [Field { name: "r_regionkey",
data_type: Int32, nullable: true, dict_id: 0, dict_is_ordered: false,
metadata: {"PARQUET:field_id": "1"} }, Field { name: "r_name",
data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: false,
metadata: {"PARQUET:field_id": "2"} }, Field { name: "r_comment",
data_type: Utf8, nullable: true, dict_id: 0, dict_is_ordered: false,
metadata: {"PARQUET:field_id": "3"} }], metadata: {} }.
This issue was likely caused by a bug in DataFusion's code. Please help
us to resolve this by filing a bug report in our issue tracker:
https://github.com/apache/datafusion/issues

## What changes are included in this PR?

PR updates `compute_properties` logic to use target (output) schema
instead of input schema. Below is example DataFusion `DataSinkExec`
implementation demonstrating that properties must be created based on
target schema, not input.


https://github.com/apache/datafusion/blob/4eacb6046773b759dae0b3d801fe8cb1c6b65c0f/datafusion/datasource/src/sink.rs#L101C1-L117C6

```rust
impl DataSinkExec {
    /// Create a plan to write to `sink`
    pub fn new(
        input: Arc<dyn ExecutionPlan>,
        sink: Arc<dyn DataSink>,
        sort_order: Option<LexRequirement>,
    ) -> Self {
        let count_schema = make_count_schema();
        let cache = Self::create_schema(&input, count_schema);
        Self {
            input,
            sink,
            count_schema: make_count_schema(),
            sort_order,
            cache,
        }
    }
....

    fn properties(&self) -> &PlanProperties {
        &self.cache
    }
```

## Are these changes tested?

Tested manually, expanded existing test to verify output schema, tested
as part of [Spice Iceberg write automated
tests](https://github.com/spiceai/spiceai/blob/trunk/crates/runtime/tests/iceberg/write/mod.rs)

Author: sgrebnov

crates/integrations/datafusion/src/physical_plan/commit.rs

@@ -57,14 +57,16 @@ impl IcebergCommitExec {
         input: Arc<dyn ExecutionPlan>,
         schema: ArrowSchemaRef,
     ) -> Self {
-        let plan_properties = Self::compute_properties(schema.clone());
+        let count_schema = Self::make_count_schema();
+
+        let plan_properties = Self::compute_properties(Arc::clone(&count_schema));
 
         Self {
             table,
             catalog,
             input,
             schema,
-            count_schema: Self::make_count_schema(),
+            count_schema,
             plan_properties,
         }
     }
@@ -469,6 +471,9 @@ mod tests {
         let commit_exec =
             IcebergCommitExec::new(table.clone(), catalog.clone(), input_exec, arrow_schema);
 
+        // Verify Execution Plan schema matches the count schema
+        assert_eq!(commit_exec.schema(), IcebergCommitExec::make_count_schema());
+
         // Execute the commit exec
         let task_ctx = Arc::new(TaskContext::default());
         let stream = commit_exec.execute(0, task_ctx)?;