Fix TSan race in blocking_context::enqueue

enqueue was calling impl_->cv.notify_one() outside the lock
scope. A foreign thread could still be inside notify_one() when
the main thread drained the queue, completed the task, saw
signal_done, and destroyed the context. TSan flagged the read
against cv during cond_signal after the waiter had released it.

Move notify_one inside the lock_guard scope, matching signal_done.

The race was latent before run() was switched to post on the
return trip: no foreign thread ever called blocking_executor::post,
so enqueue only ran on the pumping thread. testHopsBackToIoThread
exercises a compute-pool worker posting back to blocking_executor
via the return trampoline, which is what revealed the race under
TSan.

Author: sgerbino

src/test/run_blocking.cpp

@@ -87,10 +87,8 @@ void
 blocking_context::enqueue(
     std::coroutine_handle<> h)
 {
-    {
-        std::lock_guard<std::mutex> lock(impl_->mtx);
-        impl_->queue.push(h);
-    }
+    std::lock_guard<std::mutex> lock(impl_->mtx);
+    impl_->queue.push(h);
     impl_->cv.notify_one();
 }