HA entity, sensor and device classes (iteration 2, step 3)

HaEntity describes an entity for device-based discovery, throttles its
state (SEN-14) and reports availability (SEN-15); HaSensor is the
`sensor` type. HaDevice publishes discovery, availability and states
after each connect and after HA's birth message, provides the last
will and removes itself from HA. Unit tests with a fake MQTT client.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/FakeMqttClient.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <QList>
+#include <QStringList>
+
+#include "mqtt/MqttClient.h"
+
+// MqttClient for unit tests: records what is published and subscribed;
+// the test drives the connection state and incoming messages.
+class FakeMqttClient : public MqttClient {
+  Q_OBJECT
+ public:
+  struct Message {
+    QString topic;
+    QByteArray payload;
+    bool retain = false;
+  };
+
+  using MqttClient::MqttClient;
+
+  void connectToBroker() override { ++connectCalls; }
+  bool disconnectFromBroker(int) override {
+    setState(State::Disconnected);
+    return true;
+  }
+  bool publish(const QString& topic, const QByteArray& payload,
+               bool retain) override {
+    if (m_state != State::Connected) return false;
+    published.append({topic, payload, retain});
+    return true;
+  }
+  bool subscribe(const QString& topic) override {
+    if (m_state != State::Connected) return false;
+    subscriptions.append(topic);
+    return true;
+  }
+  bool waitForPublished(int) override { return true; }
+  State state() const override { return m_state; }
+
+  // Test side.
+  void setState(State state, const QString& error = {}) {
+    m_state = state;
+    emit connectionChanged(state, error);
+  }
+  void deliver(const QString& topic, const QByteArray& payload) {
+    emit messageReceived(topic, payload);
+  }
+  // The messages published to `topic`, oldest first.
+  QList<Message> to(const QString& topic) const {
+    QList<Message> result;
+    for (const Message& message : published)
+      if (message.topic == topic) result.append(message);
+    return result;
+  }
+
+  QList<Message> published;
+  QStringList subscriptions;
+  int connectCalls = 0;
+
+ private:
+  State m_state = State::Disconnected;
+};