Fix __receiver_proxy_base start_scheduler_t query.

* Return task_scheduler only when possible.
* Return nullopt in other cases.
* Terminate on exception.

Add a regression test:
  parallel_scheduler used as the start scheduler in receiver_proxy's start_schedular_t query.

Author: yulin25

include/stdexec/__detail/__task_scheduler.hpp

@@ -846,14 +846,15 @@ namespace STDEXEC
     {
       if (__value_type == __mtypeid<task_scheduler>)
       {
-        auto& __val = *static_cast<std::optional<task_scheduler>*>(__dest);
         if constexpr (__callable<get_start_scheduler_t, env_of_t<_Rcvr>>)
         {
-          __val.emplace(get_start_scheduler(get_env(__rcvr_)));
-        }
-        else
-        {
-          __val.emplace(inline_scheduler{});
+          if constexpr (__std::constructible_from<
+                          task_scheduler,
+                          __call_result_t<get_start_scheduler_t, env_of_t<_Rcvr>>>)
+          {
+            auto& __val = *static_cast<std::optional<task_scheduler>*>(__dest);
+            __val.emplace(get_start_scheduler(get_env(__rcvr_)));
+          }
         }
       }
     }

test/stdexec/schedulers/test_parallel_scheduler.cpp

@@ -94,6 +94,13 @@ TEST_CASE("trivial schedule task on parallel scheduler", "[scheduler][parallel_s
   ex::sync_wait(ex::schedule(sched));
 }
 
+TEST_CASE("can schedule from parallel scheduler to parallel scheduler",
+          "[scheduler][parallel_scheduler]")
+{
+  auto sched = ex::get_parallel_scheduler();
+  ex::sync_wait(ex::starts_on(sched, ex::starts_on(sched, ex::just())));
+}
+
 TEST_CASE("simple schedule task on parallel scheduler", "[scheduler][parallel_scheduler]")
 {
   std::thread::id             this_id = std::this_thread::get_id();