fix: avoid copy constructor instantiation in shared_ptr fallback cast (#6028)

* tests: add regressions for shared_ptr reference_internal fallback

* fix: avoid copy constructor instantiation in shared_ptr fallback cast

* Remove stray empty line

* tests: rename PyTorch shared_ptr regression test files

* refactor: add cast_non_owning helper for reference-like casts

Name the non-owning generic cast path so callers do not have to rediscover that
reference-like policies must pass null copy/move constructor callbacks. This
keeps the shared_ptr reference_internal fallback self-documenting and points
future maintainers toward the safe API.

Made-with: Cursor

* tests: guard deprecated-copy warning probes with __has_warning

Use __has_warning for the Clang-only regression test so older compiler jobs skip
unsupported warning groups instead of failing with -Wunknown-warning-option. A
simple __clang_major__ >= 13 guard would be shorter, but it bakes in a version
cutoff; __has_warning is slightly more verbose while being more robust to
vendor builds, backports, and future packaging differences.

Made-with: Cursor

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

include/pybind11/cast.h

@@ -1027,7 +1027,7 @@ public:
         }
 
         if (parent) {
-            return type_caster_base<type>::cast(
+            return type_caster_generic::cast_non_owning(
                 srcs, return_value_policy::reference_internal, parent);
         }
 

include/pybind11/detail/type_caster_base.h

@@ -1004,6 +1004,18 @@ public:
         return cast(srcs, policy, parent, copy_constructor, move_constructor, existing_holder);
     }
 
+    static handle cast_non_owning(const cast_sources &srcs,
+                                  return_value_policy policy,
+                                  handle parent,
+                                  const void *existing_holder = nullptr) {
+        // Reference-like policies alias an existing C++ object instead of creating
+        // a new one, so copy/move constructor callbacks must remain null here.
+        assert(policy == return_value_policy::reference
+               || policy == return_value_policy::reference_internal
+               || policy == return_value_policy::automatic_reference);
+        return cast(srcs, policy, parent, nullptr, nullptr, existing_holder);
+    }
+
     PYBIND11_NOINLINE static handle cast(const cast_sources &srcs,
                                          return_value_policy policy,
                                          handle parent,