feat: answer REQ with CLOSED when no relay is configured

Ported from the upstream PR (lnbits/nostrclient#74, idea from #68).
Nothing will ever serve a subscription when nostrclient has no relay
at all, so refuse it per NIP-01 instead of registering it silently.
Keyed on "no relay configured" rather than "none connected": a
configured relay that is merely reconnecting gets the cached
subscription replayed, so refusing the REQ there would be a regression.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Tbyw6FwjhEJg3gHfPHxWt

tests/test_router_ok.py

@@ -25,10 +25,14 @@ class FakeRelayManager:
     def __init__(self, relays: dict[str, FakeRelay]):
         self.relays = relays
         self.published: list[str] = []
+        self.subscriptions: list[str] = []
 
     def publish_message(self, message: str):
         self.published.append(message)
 
+    def add_subscription(self, subscription_id: str, filters):
+        self.subscriptions.append(subscription_id)
+
     def close_subscriptions(self, subscriptions):
         pass
 
@@ -139,3 +143,25 @@ async def test_stop_drops_pending_publishes(monkeypatch):
 
     assert router.pending_publishes == {}
     assert NostrRouter.received_command_results == {}
+
+
+@pytest.mark.asyncio
+async def test_req_without_configured_relays_is_closed(monkeypatch):
+    router, ws, manager = _router(monkeypatch, {})
+
+    await router._handle_client_to_nostr(json.dumps(["REQ", "sub1", {"kinds": [1]}]))
+
+    assert ws.sent == [["CLOSED", "sub1", "error: no relay connections"]]
+    assert router.subscriptions == []
+    assert manager.subscriptions == []
+
+
+@pytest.mark.asyncio
+async def test_req_with_configured_but_disconnected_relay_is_kept(monkeypatch):
+    router, ws, manager = _router(monkeypatch, {"wss://a": FakeRelay(False)})
+
+    await router._handle_client_to_nostr(json.dumps(["REQ", "sub1", {"kinds": [1]}]))
+
+    assert ws.sent == []
+    assert len(router.subscriptions) == 1
+    assert len(manager.subscriptions) == 1