[exec] on/continues_on hits __not_a_scheduler diagnostics under the debug environment when STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON

Requested by @ericniebler in #2271 — follow-up found while verifying that PR with -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON. Fails on pristine main, so it is separate from the fix in #2271.

Description

Pipelines that transform sequence senders with exec::on / exec::continues_on produce __not_a_scheduler diagnostics under extra type checking. The debug environment injected by the extra-type-checking machinery appears to shadow or alter scheduler-related queries, so the transformation's scheduler lookup sees the debug proxy environment instead of the real one and concludes there is no scheduler.

Reproduction

Compose an exec::any_sequence_sender with exec::on / exec::continues_on and build with -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON.

Suggested fix direction

Make the debug env forwarding query-aware for scheduler queries (forward get_scheduler / get_delegatee_scheduler to the wrapped environment), or exclude scheduler queries from the debug proxy.

Reproduce the failure with exec::any_sequence_sender and exec::on or exec::continues_on using -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON. Trace the extra-type-checking debug environment and its scheduler-query lookup; done means these transformations no longer produce __not_a_scheduler diagnostics while scheduler queries still reach the wrapped environment.