[SPARK-52629] Remove ArrowReaderHelper.isNestedType

### What changes were proposed in this pull request?

This PR aims to remove `ArrowReaderHelper.isNestedType` method in order to make `loadField` more flexible.

### Why are the changes needed?

This is a preparation to support more complex types. `switch` pattern is more flexible to extend `loadField` method.
In addition, `isNestedType` is used once.

### Does this PR introduce _any_ user-facing change?

No, this is an internal refactoring.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #204 from dongjoon-hyun/SPARK-52629.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Sources/SparkConnect/ArrowReader.swift

@@ -201,12 +201,15 @@ public class ArrowReader {  // swiftlint:disable:this type_body_length
   )
     -> Result<ArrowArrayHolder, ArrowError>
   {
-    if isNestedType(field.typeType) {
+    switch field.typeType {
+    case .struct_:
       return loadStructData(loadInfo, field: field)
-    } else if isFixedPrimitive(field.typeType) {
-      return loadPrimitiveData(loadInfo, field: field)
-    } else {
-      return loadVariableData(loadInfo, field: field)
+    default:
+      if isFixedPrimitive(field.typeType) {
+        return loadPrimitiveData(loadInfo, field: field)
+      } else {
+        return loadVariableData(loadInfo, field: field)
+      }
     }
   }
 

Sources/SparkConnect/ArrowReaderHelper.swift

@@ -262,15 +262,6 @@ func isFixedPrimitive(_ type: org_apache_arrow_flatbuf_Type_) -> Bool {
   }
 }
 
-func isNestedType(_ type: org_apache_arrow_flatbuf_Type_) -> Bool {
-  switch type {
-  case .struct_:
-    return true
-  default:
-    return false
-  }
-}
-
 func findArrowType(  // swiftlint:disable:this cyclomatic_complexity function_body_length
   _ field: org_apache_arrow_flatbuf_Field
 ) -> ArrowType {