GH-47252: [C++][Compute] Fix sort_indices for temporal types in arrow::Table (#50270)

### Rationale for this change
I was unable to use compute::SortIndices with timestamp type because of crash.
### What changes are included in this PR?
Fix. The issue was that comparator for merging record batches was not converting timestamp type to its physical variant. 
so it crashed on null pointer reference on [checked_cast result](https://github.com/apache/arrow/blob/d0919579cbdd37549ba163c0d61843f04bac92cd/cpp/src/arrow/compute/kernels/chunked_internal.h#L55)
### Are these changes tested?
yes
### Are there any user-facing changes?

**This PR contains a "Critical Fix".** (If the changes fix either (a) a security vulnerability, (b) a bug that caused incorrect or invalid data to be produced, or (c) a bug that causes a crash (even when the API contract is upheld), please provide explanation. If not, you can remove this.)
i've already provided, i suppose
* GitHub Issue: #47252

Lead-authored-by: Ivan Lykov <nfrmtk.aka.fkfka@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: nfrmtk <nfrmtk.aka.fkfka@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: 3 people

cpp/src/arrow/compute/kernels/vector_sort_internal.h

@@ -780,14 +780,15 @@ struct ResolvedTableSortKey {
       // so we can't simply access the column from the table directly.
       ArrayVector chunks;
       chunks.reserve(batches.size());
+      auto physical_type = GetPhysicalType(f.type->GetSharedPtr());
       int64_t null_count = 0;
       for (const auto& batch : batches) {
         ARROW_ASSIGN_OR_RAISE(auto child, f.path.GetFlattened(*batch));
         null_count += child->null_count();
-        chunks.push_back(std::move(child));
+        chunks.push_back(GetPhysicalArray(*child, physical_type));
       }
 
-      return ResolvedTableSortKey(f.type->GetSharedPtr(), std::move(chunks), f.order,
+      return ResolvedTableSortKey(physical_type, std::move(chunks), f.order,
                                   f.null_placement, null_count);
     };
 

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

@@ -1739,6 +1739,29 @@ TEST_F(TestTableSortIndices, Decimal) {
   AssertSortIndices(table, options, "[3, 4, 0, 2, 1]");
 }
 
+TEST_F(TestTableSortIndices, Timestamp) {
+  auto schema = ::arrow::schema({
+      {field("a", timestamp(TimeUnit::MICRO))},
+      {field("b", timestamp(TimeUnit::MICRO))},
+  });
+  std::vector<SortKey> sort_keys{SortKey("a", SortOrder::Ascending),
+                                 SortKey("b", SortOrder::Descending)};
+
+  auto table = TableFromJSON(schema, {R"([{"a": 1, "b": 2},
+                                          {"a": 3, "b": 4},
+                                          {"a": 8, "b": 6}
+                                          ])",
+                                      R"([{"a": 6, "b": null},
+                                          {"a": 6, "b": 7}
+                                          ])"});
+  SortOptions options(sort_keys);
+  AssertSortIndices(table, options, "[0, 1, 4, 3, 2]");
+  options.sort_keys[0].null_placement = NullPlacement::AtStart;
+  AssertSortIndices(table, options, "[0, 1, 4, 3, 2]");
+  options.sort_keys[1].null_placement = NullPlacement::AtStart;
+  AssertSortIndices(table, options, "[0, 1, 3, 4, 2]");
+}
+
 TEST_F(TestTableSortIndices, NullType) {
   auto schema = arrow::schema({
       field("a", null()),