[FIXED] MsgsTimeout iterator yields spurious (nil, nil) after a timeout - #2099

MsgsTimeout returns a range-over-func iterator. When NextMsg times out it
yields (nil, ErrTimeout). ErrTimeout is not fatal, so a consumer is allowed
to keep iterating (e.g. to wait for more messages or do periodic work between
batches). In that case the loop falls through to yield(msg, nil) with
msg == nil and err == nil, so the consumer gets a spurious (nil, nil) pair
after every timeout. Code that treats err == nil as "got a message" then
operates on a nil *Msg.

The Msgs() iterator returns after any error and doesn't have this. The fix is
a continue after the non-fatal timeout branch so it loops back to NextMsg
instead of falling through.

The existing with timeout test didn't catch it because it breaks on the first
timeout, so the iterator returns before reaching the second yield. The added
test keeps iterating across timeouts:

# before the fix
--- FAIL: TestSubscribeIterator/timeout_keeps_yielding_only_ErrTimeout
    nats_iter_test.go:105: iteration 1: got msg=<nil> with nil error, expected ErrTimeout
# after
ok  github.com/nats-io/nats.go/test

TestSubscribeIterator and TestQueueSubscribeIterator pass with -race.

AI assistance disclosure: this fix and test were prepared with the help of an AI
coding assistant; I reviewed and verified the change and tests myself.

Signed-off-by: Sueun Cho sueun.dev@gmail.com