fix: suppress -Wredundant-decls for class.h declarations

Several functions are forward-declared in headers that cannot include
class.h; GCC's -Wredundant-decls (used in CI cxx_flags) flags the
second declaration.

Assisted-by: ClaudeCode:claude-fable-5

include/pybind11/detail/class.h

diff --git a/include/pybind11/detail/class.h b/include/pybind11/detail/class.h
index 2ecde4f..74e5ef0 100644
--- a/include/pybind11/detail/class.h
+++ b/include/pybind11/detail/class.h
@@ -27,6 +27,12 @@
         setattr((PyObject *) obj, "__qualname__", nameobj)
 #endif
 
+PYBIND11_WARNING_PUSH
+// Several of these functions are forward-declared in other headers (internals.h,
+// type_caster_base.h, trampoline_self_life_support.h, cpp_conduit.h), which cannot
+// include this file; the declarations here are the canonical set.
+PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
+
 std::string get_fully_qualified_tp_name(PyTypeObject *type);
 
 PyTypeObject *type_incref(PyTypeObject *type);
@@ -124,13 +130,8 @@
 /// to destroy the C++ object itself, while the rest is Python bookkeeping.
 extern "C" void pybind11_object_dealloc(PyObject *self);
 
-PYBIND11_WARNING_PUSH
-PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
-
 std::string error_string();
 
-PYBIND11_WARNING_POP
-
 /** Create the type which can be used as a common base for all classes.  This is
     needed in order to satisfy Python's requirements for multiple inheritance.
     Return value: New reference. */
@@ -158,6 +159,8 @@
     Return value: New reference. */
 PyObject *make_new_python_type(const type_record &rec);
 
+PYBIND11_WARNING_POP
+
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)