ORC-1881: [C++] Populate dstBatch's scale and precision in DecimalConvertColumnReader

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

Set dstBatch's decimal and precision when `DecimalConvertColumnReader::next`.
Fix ORC-1881.

### Why are the changes needed?

During decimal-to-decimal conversion in `SchemaEvolution`, the target decimal's scale and precision are incorrectly initialized to zero, producing a corrupted `ColumnVectorBatch`.

### How was this patch tested?

Unit test.

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

No.

Closes #2197 from kecookier/backport-2194-branch-2.0.

Authored-by: zhaokuo03 <zhaokuo03@meituan.com>
Signed-off-by: Gang Wu <ustcwg@gmail.com>

Author: zhaokuo03

c++/src/ConvertColumnReader.cc

@@ -558,6 +558,8 @@ namespace orc {
 
       const auto& srcBatch = *SafeCastBatchTo<const FileTypeBatch*>(data.get());
       auto& dstBatch = *SafeCastBatchTo<ReadTypeBatch*>(&rowBatch);
+      dstBatch.precision = toPrecision;
+      dstBatch.scale = toScale;
       for (uint64_t i = 0; i < numValues; ++i) {
         if (!rowBatch.hasNulls || rowBatch.notNull[i]) {
           convertDecimalToDecimal(dstBatch, i, srcBatch);

c++/test/TestConvertColumnReader.cc

@@ -650,6 +650,10 @@ namespace orc {
     auto& readC2 = dynamic_cast<Decimal128VectorBatch&>(*readStructBatch.fields[1]);
     auto& readC3 = dynamic_cast<Decimal64VectorBatch&>(*readStructBatch.fields[2]);
     auto& readC4 = dynamic_cast<Decimal128VectorBatch&>(*readStructBatch.fields[3]);
+    EXPECT_TRUE(9 == readC1.precision && 5 == readC1.scale);
+    EXPECT_TRUE(20 == readC2.precision && 5 == readC2.scale);
+    EXPECT_TRUE(10 == readC3.precision && 3 == readC3.scale);
+    EXPECT_TRUE(19 == readC4.precision && 3 == readC4.scale);
     EXPECT_EQ(TEST_CASES, readBatch->numElements);
     for (int i = 0; i < TEST_CASES / 2; i++) {
       size_t idx = static_cast<size_t>(i);