GH-46825: [R] Use smallest_decimal() from C++ instead of working out which decimal type to instantiate in R (#47906)

### Rationale for this change

R code duplicated code from C++

### What changes are included in this PR?

Remove duplication

### Are these changes tested?

There were existing tests

### Are there any user-facing changes?

Nah
* GitHub Issue: #46825

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

Author: thisisnic

r/R/arrowExports.R

@@ -604,16 +604,7 @@ timestamp <- function(unit = c("s", "ms", "us", "ns"), timezone = "") {
 #' @export
 decimal <- function(precision, scale) {
   args <- check_decimal_args(precision, scale)
-
-  if (args$precision > 38) {
-    decimal256(args$precision, args$scale)
-  } else if (args$precision > 18) {
-    decimal128(args$precision, args$scale)
-  } else if (args$precision > 9) {
-    decimal64(args$precision, args$scale)
-  } else {
-    decimal32(args$precision, args$scale)
-  }
+  SmallestDecimal__initialize(args$precision, args$scale)
 }
 
 #' @rdname data-type

r/R/type.R

@@ -213,6 +213,12 @@ std::shared_ptr<arrow::DataType> Decimal256Type__initialize(int32_t precision,
   return ValueOrStop(arrow::Decimal256Type::Make(precision, scale));
 }
 
+// [[arrow::export]]
+std::shared_ptr<arrow::DataType> SmallestDecimal__initialize(int32_t precision,
+                                                             int32_t scale) {
+  return arrow::smallest_decimal(precision, scale);
+}
+
 // [[arrow::export]]
 std::shared_ptr<arrow::DataType> DayTimeInterval__initialize() {
   return arrow::day_time_interval();