[FEA] Add java tests for SUM_OVERFLOW reduction on signed integers and decimals

**Is your feature request related to a problem? Please describe.**
#22465 extends the C++ `SUM_OVERFLOW` reduction from being INT64-only to supporting all signed integer types (`INT8`, `INT16`, `INT32`, `INT64`) as well as all decimal types (`DECIMAL32`, `DECIMAL64`, `DECIMAL128`). The Java bindings already expose this aggregation, but the existing tests were written under the previous INT64-only assumption.

**Describe the solution you'd like**
_originally proposed by @mythrocks at https://github.com/rapidsai/cudf/pull/22465#discussion_r3269949375_:

In https://github.com/rapidsai/cudf/blob/main/java/src/test/java/ai/rapids/cudf/ReductionTest.java

1. Generalize the `SumOverflowResult` / `readSumOverflow` helper so the result's sum child can be read for non-INT64 types.
2. Add `testSumOverflowSignedIntegralTypes` covering non-overflowing cases for `INT8`, `INT16`, `INT32`, `INT64`.
3. Add `testSumOverflowSignedDecimalTypes` covering non-overflowing cases for `DECIMAL32`, `DECIMAL64`, `DECIMAL128`.

Both new methods should call the generalized helper to verify the overflow flag is false and the sum child matches the expected value.