Bug: Mqtt fix testcase due to immediately sending MapReport (#8872) (#9784)

* Add ESP32 Power Management lessons learned document

Documents our experimentation with ESP-IDF DFS and why it doesn't
work well for Meshtastic (RTOS locks, BLE locks, USB issues).

Proposes simpler alternative: manual setCpuFrequencyMhz() control
with explicit triggers for when to go fast vs slow.

* Added a lambda function to clear startup output in the MQTT unit test to ensure a clean state before and after the MQTT subscription process.

test/test_mqtt/MQTT.cpp

@@ -289,13 +289,23 @@ class MQTTUnitTest : public MQTT
         mqtt = unitTest = new MQTTUnitTest();
         mqtt->start();
 
+        auto clearStartupOutput = []() {
+            pubsub->published_.clear();
+            if (mockMeshService != nullptr) {
+                mockMeshService->messages_.clear();
+                mockMeshService->notifications_.clear();
+            }
+        };
+
         if (!moduleConfig.mqtt.enabled || moduleConfig.mqtt.proxy_to_client_enabled || *moduleConfig.mqtt.root) {
             loopUntil([] { return true; }); // Loop once
+            clearStartupOutput();
             return;
         }
         // Wait for MQTT to subscribe to all topics.
         TEST_ASSERT_TRUE(loopUntil(
             [] { return pubsub->subscriptions_.count("msh/2/e/test/+") && pubsub->subscriptions_.count("msh/2/e/PKI/+"); }));
+        clearStartupOutput();
     }
     PubSubClient &getPubSub() { return pubSub; }
 };
@@ -930,4 +940,4 @@ void setup()
     UNITY_END();
 }
 #endif
-void loop() {}
+void loop() {}