fix(ring): serialize capacity and shutdown checks

[bensynapse]

I run Live Tennis API.

WithQueueSize(2) can admit four jobs when producers submit at the same time. Ring.Queue checks capacity before locking the buffer.
Ring.Request also reads count without locking during shutdown. Concurrent readers can trigger a data race and return ErrNoTaskInQueue after closure.

Move both checks inside the existing mutex so they share the lock with buffer updates. Add concurrent capacity and shutdown drain tests.

make test passes with race detection on Go 1.26.8 and 1.27.0. The focused regressions pass 50 runs with -race.
make lint, go build ./... and go vet ./... pass. Both example modules build and execute successfully.