[SPARK-54552][CONNECT] Fix SparkConnectResultSet.getString to handle BINARY data type with UTF_8

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

Fixed `SparkConnectResultSet.getString()` to properly convert BINARY data to UTF-8 strings instead of returning byte array object references (e.g., "[B<hashcode>").

### Why are the changes needed?

The current implementation violates JDBC specification behavior. Users calling getString() on BINARY columns expect UTF-8 decoded strings, not Java object references.

Before
```
SELECT binary('xDeAdBeEf')

spark-sql: `\xDeAdBeEf`
beeline with STS: `\xDeAdBeEf`
beeline with Connect Server: `[B4d518c66`
```

After
```
SELECT binary('xDeAdBeEf')

spark-sql: `\xDeAdBeEf`
beeline with STS: `\xDeAdBeEf`
beeline with Connect Server: `\xDeAdBeEf`
```

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

Yes. getString() on BINARY columns now returns UTF-8 decoded strings instead of byte array references like "[B1a2b3c4d".

### How was this patch tested?

Added new test

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

No

Closes #53262 from vinodkc/br_fix_getString_BINARY.

Authored-by: vinodkc <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>
(cherry picked from commit f5b9ea8)
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: vinodkc

sql/connect/client/jdbc/src/main/scala/org/apache/spark/sql/connect/client/jdbc/SparkConnectResultSet.scala

@@ -114,7 +114,13 @@ class SparkConnectResultSet(
   }
 
   override def getString(columnIndex: Int): String = {
-    getColumnValue(columnIndex, null: String) { idx => String.valueOf(currentRow.get(idx)) }
+    getColumnValue(columnIndex, null: String) { idx =>
+      currentRow.get(idx) match {
+        case bytes: Array[Byte] =>
+          new String(bytes, java.nio.charset.StandardCharsets.UTF_8)
+        case other => String.valueOf(other)
+      }
+    }
   }
 
   override def getBoolean(columnIndex: Int): Boolean = {

sql/connect/client/jdbc/src/test/scala/org/apache/spark/sql/connect/client/jdbc/SparkConnectJdbcDataTypeSuite.scala

@@ -385,6 +385,11 @@ class SparkConnectJdbcDataTypeSuite extends ConnectFunSuite with RemoteSparkSess
       assert(bytes.length === testBytes.length)
       assert(bytes.sameElements(testBytes))
       assert(!rs.wasNull)
+
+      val stringValue = rs.getString(1)
+      val expectedString = new String(testBytes, java.nio.charset.StandardCharsets.UTF_8)
+      assert(stringValue === expectedString)
+
       assert(!rs.next())
 
       val metaData = rs.getMetaData
@@ -396,6 +401,22 @@ class SparkConnectJdbcDataTypeSuite extends ConnectFunSuite with RemoteSparkSess
     }
   }
 
+  test("get binary type with UTF-8 text") {
+    val textBytes = "\\xDeAdBeEf".getBytes(java.nio.charset.StandardCharsets.UTF_8)
+    val hexString = textBytes.map(b => "%02X".format(b)).mkString
+    withExecuteQuery(s"SELECT CAST(X'$hexString' AS BINARY)") { rs =>
+      assert(rs.next())
+      val bytes = rs.getBytes(1)
+      assert(bytes !== null)
+      assert(bytes.sameElements(textBytes))
+
+      val stringValue = rs.getString(1)
+      assert(stringValue === "\\xDeAdBeEf")
+
+      assert(!rs.next())
+    }
+  }
+
   test("get binary type with null") {
     withExecuteQuery("SELECT cast(null as binary)") { rs =>
       assert(rs.next())
@@ -437,6 +458,9 @@ class SparkConnectJdbcDataTypeSuite extends ConnectFunSuite with RemoteSparkSess
       assert(bytes !== null)
       assert(bytes.length === 0)
       assert(!rs.wasNull)
+
+      val stringValue = rs.getString(1)
+      assert(stringValue === "")
       assert(!rs.next())
     }
   }