executors: Replace tag_invoke(bulk_t) with member function on executor_scheduler

Refactor the P2300 bulk customization point from a deprecated tag_invoke
free function to a member function on executor_scheduler, matching the
modern P2300 sender/receiver specification.

- Add bulk() member declaration in executor_scheduler (executor_scheduler.hpp)
- Define bulk() out-of-line in executor_scheduler_bulk.hpp after
  executor_bulk_sender is fully defined
- Remove the tag_invoke(bulk_t, executor_scheduler, ...) free function

Signed-off-by: Shivansh Singh <your_github_email@example.com>

Author: Shivansh Singh

libs/core/executors/include/hpx/executors/executor_scheduler.hpp

@@ -199,5 +199,10 @@ namespace hpx::execution::experimental {
         {
             return {HPX_MOVE(exec_)};
         }
+
+        // P2300 bulk customization -- defined out-of-line in
+        // executor_scheduler_bulk.hpp after executor_bulk_sender is available.
+        template <typename Sender, typename Shape, typename F>
+        auto bulk(Sender&& sender, Shape const& shape, F&& f) const;
     };
 }    // namespace hpx::execution::experimental

libs/core/executors/include/hpx/executors/executor_scheduler_bulk.hpp

@@ -162,12 +162,14 @@ namespace hpx::execution::experimental {
         };
     }    // namespace detail
 
-    template <typename Executor, typename Sender, typename Shape, typename F>
-    auto tag_invoke(bulk_t, executor_scheduler<Executor> const& sched,
-        Sender&& sender, Shape const& shape, F&& f)
+    // Out-of-line definition for executor_scheduler::bulk() member.
+    template <typename Executor>
+    template <typename Sender, typename Shape, typename F>
+    auto executor_scheduler<Executor>::bulk(
+        Sender&& sender, Shape const& shape, F&& f) const
     {
         return detail::executor_bulk_sender<Executor, std::decay_t<Sender>,
             Shape, std::decay_t<F>>{
-            sched.exec_, HPX_FORWARD(Sender, sender), shape, HPX_FORWARD(F, f)};
+            exec_, HPX_FORWARD(Sender, sender), shape, HPX_FORWARD(F, f)};
     }
 }    // namespace hpx::execution::experimental