[SPARK-47307][SQL][FOLLOWUP] Promote spark.sql.legacy.chunkBase64Stri…

…ng.enabled from a legacy/internal config to a regular/public one

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

+ Promote spark.sql.legacy.chunkBase64String.enabled from a legacy/internal config to a regular/public one.
+ Add test cases for unbase64

### Why are the changes needed?

Keep the same behavior as before. More details: apache#47303 (comment)

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

yes, revert behavior change introduced in apache#47303

### How was this patch tested?

existing unit test

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

No

Closes apache#47410 from wForget/SPARK-47307_followup.

Lead-authored-by: wforget <643348094@qq.com>
Co-authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>

connect/common/src/test/resources/query-tests/explain-results/function_base64.explain

@@ -1,2 +1,2 @@
-Project [static_invoke(Base64.encode(cast(g#0 as binary), false)) AS base64(CAST(g AS BINARY))#0]
+Project [static_invoke(Base64.encode(cast(g#0 as binary), true)) AS base64(CAST(g AS BINARY))#0]
 +- LocalRelation <empty>, [id#0L, a#0, b#0, d#0, e#0, f#0, g#0]

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -3525,14 +3525,13 @@ object SQLConf {
     .booleanConf
     .createWithDefault(false)
 
-  val CHUNK_BASE64_STRING_ENABLED = buildConf("spark.sql.legacy.chunkBase64String.enabled")
-    .internal()
+  val CHUNK_BASE64_STRING_ENABLED = buildConf("spark.sql.chunkBase64String.enabled")
     .doc("Whether to truncate string generated by the `Base64` function. When true, base64" +
       " strings generated by the base64 function are chunked into lines of at most 76" +
       " characters. When false, the base64 strings are not chunked.")
     .version("3.5.2")
     .booleanConf
-    .createWithDefault(false)
+    .createWithDefault(true)
 
   val ENABLE_DEFAULT_COLUMNS =
     buildConf("spark.sql.defaultColumn.enabled")

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/StringExpressionsSuite.scala

@@ -520,6 +520,10 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     withSQLConf(SQLConf.CHUNK_BASE64_STRING_ENABLED.key -> "true") {
       checkEvaluation(Base64(Literal(longString.getBytes)), chunkEncoded)
     }
+
+    // check if unbase64 works well for chunked and non-chunked encoded strings
+    checkEvaluation(StringDecode(UnBase64(Literal(encoded)), Literal("utf-8")), longString)
+    checkEvaluation(StringDecode(UnBase64(Literal(chunkEncoded)), Literal("utf-8")), longString)
   }
 
   test("initcap unit test") {