fix(exec): mark debug trial connects as root connects

With STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON, the debug machinery's trial
connect uses a debug environment that, unlike every real connect root
(sync_wait, start_detached, start_now), did not answer the __root_t
query. on()'s transform_sender relies on that query to decide whether it
may fall back to an inline_scheduler when the environment has no
scheduler to restore, so senders like on(sch, closure) over just(...)
would trip __not_a_scheduler diagnostics during the extra type checking
of sequence pipelines.

The debug environment now includes __root_env, since the trial connect
it performs is genuinely a root connect. Additionally, __root_t is now a
forwarding query so that rootness survives environment adaptation such
as write_env, matching the semantics that a connect tree's root
property holds for every operation within it.

With this change, all sequence test files compile cleanly with
STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON. Fixes #2274.

Author: alwaysprince05

include/stdexec/__detail/__debug.hpp

@@ -78,7 +78,7 @@ namespace STDEXEC
 
     STDEXEC_MODULE_EXPORT_AUTHORING
     template <class _Env>
-    using __env_t = env<prop<__debug_env_t, bool>, _Env>;
+    using __env_t = env<prop<__debug_env_t, bool>, __root_env, _Env>;
 
     STDEXEC_MODULE_EXPORT_AUTHORING
     template <class _CvSender, class _Env, class... _Sigs>

include/stdexec/__detail/__env.hpp

@@ -142,10 +142,13 @@ namespace STDEXEC
 
     struct __root_t : __query<__root_t>
     {
+      // This is a forwarding query so that the "rootness" of an environment survives
+      // being wrapped by environment-adapting senders like `write_env`. Rootness is a
+      // property of the root of a connect tree; it holds for every operation within it.
       STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)
       static consteval auto query(forwarding_query_t) noexcept -> bool
       {
-        return false;
+        return true;
       }
     };