Add NOLINTNEXTLINE(bugprone-unused-return-value) in smart_holder_poc_test.cpp

Follow-on to #5272 — clang-tidy upgrade (to version 18)

Fixes this error:

```
/__w/pybind11/pybind11/tests/pure_cpp/smart_holder_poc_test.cpp:167:12: error: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value,-warnings-as-errors]
  167 |     (void) new_owner.release(); // Manually verified: without this, clang++ -fsanitize=address
      |            ^~~~~~~~~~~~~~~~~~~
```

See also: https://clang.llvm.org/extra/clang-tidy/checks/bugprone/unused-return-value.html

Specifically there:

> std::unique_ptr::release(). Not using the return value can lead to resource leaks if the same pointer isn’t stored anywhere else. Often, ignoring the release() return value indicates that the programmer confused the function with reset().

I.e. the only way to tell clang-tidy that the smart_holder_poc_test.cpp is correct is to add the `NOLINT`.

Author: rwgk

tests/pure_cpp/smart_holder_poc_test.cpp

@@ -164,6 +164,7 @@ TEST_CASE("from_raw_ptr_take_ownership+disown+reclaim_disowned", "[S]") {
     REQUIRE(*new_owner == 19);
     hld.reclaim_disowned(); // Manually veriified: without this, clang++ -fsanitize=address reports
                             // "detected memory leaks".
+    // NOLINTNEXTLINE(bugprone-unused-return-value)
     (void) new_owner.release(); // Manually verified: without this, clang++ -fsanitize=address
                                 // reports "attempting double-free".
     REQUIRE(hld.as_lvalue_ref<int>() == 19);