bridge: the discovery answer is published through send()

The answer on <root>/discover_r called the publisher directly; a publisher
that threw or rejected was only caught by the handler meant for listeners
of "discover". It now takes the path of every device message: send()
resolves, and a failure goes to the "error" event when somebody listens.
A test covers a publish that fails, throws and rejects, with and without
an "error" listener: receive() resolves each time and "discover" is not
emitted.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/transport.test.js

@@ -63,3 +63,29 @@ test("unregisterDevice and clearDevices take the publisher from the device", ()
   bridge.clearDevices();
   assert.equal(plug.publisher, null);
 });
+
+test("a discovery answer that cannot be published: receive() resolves and the \"error\" event says why", async () => {
+  const gone = new MemoryPublisher();
+  gone.failWith = new Error("broker gone");
+  const publishers = {
+    "fails": gone,
+    "throws": { publish() { throw new Error("publisher threw"); } },
+    "rejects": { publish: () => Promise.reject(new Error("publisher rejected")) },
+  };
+  const expected = { "fails": "broker gone", "throws": "publisher threw", "rejects": "publisher rejected" };
+  for (const [how, publisher] of Object.entries(publishers)) {
+    const bridge = new Alex2MQTT("user", "password", "root", false, { publisher });
+    bridge.addDevice({ endpointId: "lamp-1", name: "Lamp", categories: ["LIGHT"] }).add(PowerController);
+    const errors = [];
+    const discovered = [];
+    bridge.on("error", (err) => errors.push(err.message));
+    bridge.on("discover", (n) => discovered.push(n));
+    await bridge.receive("root/discover", "");
+    assert.deepEqual(errors, [expected[how]], how);
+    assert.deepEqual(discovered, [], how);
+
+    // Without a listener the error has nowhere to go, and still nothing is thrown or rejected
+    bridge.removeAllListeners("error");
+    await bridge.receive("root/discover", "");
+  }
+});