[SPARK-47904][SQL] Preserve case in Avro schema when using enableStab…

…leIdentifiersForUnionType

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

When `enableStableIdentifiersForUnionType` is enabled, all of the types are lowercased which creates a problem when field types are case-sensitive:

Union type with fields:
```
Schema.createEnum("myENUM", "", null, List[String]("E1", "e2").asJava),
Schema.createRecord("myRecord2", "", null, false, List[Schema.Field](new Schema.Field("F", Schema.create(Type.FLOAT))).asJava)
```

would become

```
struct<member_myenum: string, member_myrecord2: struct<f: float>>
```

but instead should be
```
struct<member_myENUM: string, member_myRecord2: struct<F: float>>
```

### Why are the changes needed?

Fixes a bug of lowercasing the field name (the type portion).

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

Yes, if a user enables `enableStableIdentifiersForUnionType` and has Union types, all fields will preserve the case. Previously, the field names would be all in lowercase.

### How was this patch tested?

I added a test case to verify the new field names.

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

No.

Closes apache#46126 from sadikovi/SPARK-47904.

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

connector/avro/src/main/scala/org/apache/spark/sql/avro/SchemaConverters.scala

@@ -208,14 +208,12 @@ object SchemaConverters {
                   // could be "a" and "A" and we need to distinguish them. In this case, we throw
                   // an exception.
                   // Stable id prefix can be empty so the name of the field can be just the type.
-                  val tempFieldName =
-                    s"${stableIdPrefixForUnionType}${s.getName.toLowerCase(Locale.ROOT)}"
-                  if (fieldNameSet.contains(tempFieldName)) {
+                  val tempFieldName = s"${stableIdPrefixForUnionType}${s.getName}"
+                  if (!fieldNameSet.add(tempFieldName.toLowerCase(Locale.ROOT))) {
                     throw new IncompatibleSchemaException(
-                      "Cannot generate stable indentifier for Avro union type due to name " +
+                      "Cannot generate stable identifier for Avro union type due to name " +
                       s"conflict of type name ${s.getName}")
                   }
-                  fieldNameSet.add(tempFieldName)
                   tempFieldName
                 } else {
                   s"member$i"

connector/avro/src/test/scala/org/apache/spark/sql/avro/AvroSuite.scala

@@ -377,7 +377,7 @@ abstract class AvroSuite
           "",
           Seq())
       }
-      assert(e.getMessage.contains("Cannot generate stable indentifier"))
+      assert(e.getMessage.contains("Cannot generate stable identifier"))
     }
     {
       val e = intercept[Exception] {
@@ -388,7 +388,7 @@ abstract class AvroSuite
           "",
           Seq())
       }
-      assert(e.getMessage.contains("Cannot generate stable indentifier"))
+      assert(e.getMessage.contains("Cannot generate stable identifier"))
     }
     // Two array types or two map types are not allowed in union.
     {
@@ -441,6 +441,33 @@ abstract class AvroSuite
     }
   }
 
+  test("SPARK-47904: Test that field name case is preserved") {
+    checkUnionStableId(
+      List(
+        Schema.createEnum("myENUM", "", null, List[String]("E1", "e2").asJava),
+        Schema.createRecord("myRecord", "", null, false,
+          List[Schema.Field](new Schema.Field("f", Schema.createFixed("myField", "", null, 6)))
+            .asJava),
+        Schema.createRecord("myRecord2", "", null, false,
+          List[Schema.Field](new Schema.Field("F", Schema.create(Type.FLOAT)))
+            .asJava)),
+      "struct<member_myENUM: string, member_myRecord: struct<f: binary>, " +
+                    "member_myRecord2: struct<F: float>>",
+      Seq())
+
+    {
+      val e = intercept[Exception] {
+        checkUnionStableId(
+          List(
+            Schema.createRecord("myRecord", "", null, false, List[Schema.Field]().asJava),
+            Schema.createRecord("myrecord", "", null, false, List[Schema.Field]().asJava)),
+          "",
+          Seq())
+      }
+      assert(e.getMessage.contains("Cannot generate stable identifier"))
+    }
+  }
+
   test("SPARK-46930: Use custom prefix for stable ids when converting Union type") {
     // Test default "member_" prefix.
     checkUnionStableId(