fix: Reserved sort order ID cannot contain any fields (#1978)

## Which issue does this PR close?

- Closes #1963.

## What changes are included in this PR?

This change validates that table metadata with reserved sort order ID
(0) cannot contain fields associated with it. If this is found, we error
out instead of silently parsing arbitrary field values.

## Are these changes tested?

Added the unit test described in the issue and verified that the check
is now enforced.

Author: aditya-subrahmanyan

crates/iceberg/src/spec/table_metadata.rs

@@ -506,6 +506,19 @@ impl TableMetadata {
 
     /// If the default sort order is unsorted but the sort order is not present, add it
     fn try_normalize_sort_order(&mut self) -> Result<()> {
+        // Validate that sort order ID 0 (reserved for unsorted) has no fields
+        if let Some(sort_order) = self.sort_order_by_id(SortOrder::UNSORTED_ORDER_ID)
+            && !sort_order.fields.is_empty()
+        {
+            return Err(Error::new(
+                ErrorKind::Unexpected,
+                format!(
+                    "Sort order ID {} is reserved for unsorted order",
+                    SortOrder::UNSORTED_ORDER_ID
+                ),
+            ));
+        }
+
         if self.sort_order_by_id(self.default_sort_order_id).is_some() {
             return Ok(());
         }
@@ -3795,4 +3808,57 @@ mod tests {
         assert!(final_metadata.name_exists_in_any_schema("new_field")); // only in current schema
         assert!(!final_metadata.name_exists_in_any_schema("never_existed"));
     }
+
+    #[test]
+    fn test_invalid_sort_order_id_zero_with_fields() {
+        let metadata = r#"
+        {
+            "format-version": 2,
+            "table-uuid": "9c12d441-03fe-4693-9a96-a0705ddf69c1",
+            "location": "s3://bucket/test/location",
+            "last-sequence-number": 111,
+            "last-updated-ms": 1600000000000,
+            "last-column-id": 3,
+            "current-schema-id": 1,
+            "schemas": [
+                {
+                    "type": "struct",
+                    "schema-id": 1,
+                    "fields": [
+                        {"id": 1, "name": "x", "required": true, "type": "long"},
+                        {"id": 2, "name": "y", "required": true, "type": "long"}
+                    ]
+                }
+            ],
+            "default-spec-id": 0,
+            "partition-specs": [{"spec-id": 0, "fields": []}],
+            "last-partition-id": 999,
+            "default-sort-order-id": 0,
+            "sort-orders": [
+                {
+                    "order-id": 0,
+                    "fields": [
+                        {
+                            "transform": "identity",
+                            "source-id": 1,
+                            "direction": "asc",
+                            "null-order": "nulls-first"
+                        }
+                    ]
+                }
+            ],
+            "properties": {},
+            "current-snapshot-id": -1,
+            "snapshots": []
+        }
+        "#;
+
+        let result: Result<TableMetadata, serde_json::Error> = serde_json::from_str(metadata);
+
+        // Should fail because sort order ID 0 is reserved for unsorted order and cannot have fields
+        assert!(
+            result.is_err(),
+            "Parsing should fail for sort order ID 0 with fields"
+        );
+    }
 }