GH-50524: [C++] Honor array offset in pairwise_diff (#50858)

### Rationale for this change

`ArraySpan::SetSlice` replaces the offset. `pairwise_diff` copied the input span and then sliced from `left_start`/`right_start`, so a sliced array was read from the parent buffer. With `[99, 1, 4, 9, 16]` sliced to `[1, 4, 9, 16]`, period=1 produced `[-98, 3, 5, 7]` instead of `[null, 3, 5, 7]`.

### What changes are included in this PR?

The kernel passes `input.offset + left_start` (and the same for the right side). The regression test uses a sliced int64 array for both period signs, on `pairwise_diff` and `pairwise_diff_checked`.

### Are these changes tested?

Yes, `TestPairwiseDiff.SlicedInput` in `vector_pairwise_test.cc`.

### Are there any user-facing changes?

`pairwise_diff` on a sliced array now diffs the sliced values. Callers who passed a slice and got parent-buffer values will see different (correct) output.

* GitHub Issue: #50524

Authored-by: Krishnanand G <118352827+Krishnanand-G@users.noreply.github.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: Krishnanand-G

cpp/src/arrow/compute/kernels/vector_pairwise.cc

@@ -73,10 +73,12 @@ Status PairwiseExecImpl(KernelContext* ctx, const ArraySpan& input,
   }
   result->null_count = null_count;
   // prepare input span
+  // SetSlice overwrites offset. Keep the input's offset so a sliced
+  // array is not read from the start of the parent buffer.
   ArraySpan left(input);
-  left.SetSlice(left_start, computed_length);
+  left.SetSlice(input.offset + left_start, computed_length);
   ArraySpan right(input);
-  right.SetSlice(right_start, computed_length);
+  right.SetSlice(input.offset + right_start, computed_length);
   // prepare output span
   ArraySpan output_span;
   output_span.SetMembers(*result);

cpp/src/arrow/compute/kernels/vector_pairwise_test.cc

@@ -151,6 +151,26 @@ TEST_F(TestPairwiseDiff, Numeric) {
   }
 }
 
+TEST_F(TestPairwiseDiff, SlicedInput) {
+  // Slice() keeps a nonzero offset into the parent buffer. The kernel
+  // used to treat that offset as zero and read values before the slice.
+  auto base = ArrayFromJSON(int64(), "[99, 1, 4, 9, 16, 88]");
+  auto sliced = base->Slice(1, 4);
+
+  {
+    PairwiseOptions options(1);
+    auto expected = ArrayFromJSON(int64(), "[null, 3, 5, 7]");
+    CheckVectorUnary("pairwise_diff", sliced, expected, &options);
+    CheckVectorUnary("pairwise_diff_checked", sliced, expected, &options);
+  }
+  {
+    PairwiseOptions options(-1);
+    auto expected = ArrayFromJSON(int64(), "[-3, -5, -7, null]");
+    CheckVectorUnary("pairwise_diff", sliced, expected, &options);
+    CheckVectorUnary("pairwise_diff_checked", sliced, expected, &options);
+  }
+}
+
 TEST_F(TestPairwiseDiff, Overflow) {
   {
     PairwiseOptions options(1);