Refactor RleRunDecoder::GetBatchWithCount to avoid code duplication

Address review feedback by calling GetBatch instead of duplicating the
fill logic. For RLE runs, counting remains O(1) since all values in the
run are identical.

Author: tennisleng

cpp/src/arrow/util/rle_encoding_internal.h

@@ -308,20 +308,17 @@ class RleRunDecoder {
     return to_read;
   }
 
-  /// Get a batch of values and count how many equal match_value
+  /// Get a batch of values and count how many equal match_value.
+  /// For RLE runs, counting is O(1) because all values in the run are identical.
   [[nodiscard]] rle_size_t GetBatchWithCount(value_type* out, rle_size_t batch_size,
                                              rle_size_t value_bit_width,
                                              value_type match_value, int64_t* out_count) {
-    if (ARROW_PREDICT_FALSE(remaining_count_ == 0)) {
-      return 0;
-    }
-
-    const auto to_read = std::min(remaining_count_, batch_size);
-    std::fill(out, out + to_read, value_);
-    if (value_ == match_value) {
+    // Save value_ before GetBatch since it doesn't change during the call.
+    const auto current_value = value_;
+    const auto to_read = GetBatch(out, batch_size, value_bit_width);
+    if (current_value == match_value) {
       *out_count += to_read;
     }
-    remaining_count_ -= to_read;
     return to_read;
   }