Fix listener memory allocation bug.

- Replaced the buggy, oldschool, plain pointer based list approach with more robust FreeRTOS linked list implementation for the listener API. Fixes #1049.

- Added relevant test cases. Refs #1030.

test_apps/main/test_relay_chn.c

@@ -6,6 +6,7 @@
 #include <freertos/FreeRTOS.h>
 #include <freertos/task.h>
 #include "sdkconfig.h" // For accessing CONFIG_* values
+#include <string.h>
 
 // Test GPIOs and channel IDs
 // Please ensure these GPIOs are correct and suitable for your board.
@@ -242,6 +243,127 @@ TEST_CASE("FREE to Running transition without inertia", "[relay_chn][inertia]")
     TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FORWARD, relay_chn_get_state(ch));
 }
 
+// --- Listener Test Globals ---
+typedef struct {
+    uint8_t chn_id;
+    relay_chn_state_t old_state;
+    relay_chn_state_t new_state;
+    int call_count;
+} listener_callback_info_t;
+
+static listener_callback_info_t listener1_info;
+static listener_callback_info_t listener2_info;
+
+// --- Listener Test Helper Functions ---
+
+// Clear the memory from possible garbage values
+static void reset_listener_info(listener_callback_info_t* info) {
+    memset(info, 0, sizeof(listener_callback_info_t));
+}
+
+static void test_listener_1(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state) {
+    listener1_info.chn_id = chn_id;
+    listener1_info.old_state = old_state;
+    listener1_info.new_state = new_state;
+    listener1_info.call_count++;
+}
+
+static void test_listener_2(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state) {
+    listener2_info.chn_id = chn_id;
+    listener2_info.old_state = old_state;
+    listener2_info.new_state = new_state;
+    listener2_info.call_count++;
+}
+
+// ### Listener Functionality Tests
+
+TEST_CASE("Listener is called on state change", "[relay_chn][listener]") {
+    uint8_t ch = 0;
+    reset_listener_info(&listener1_info);
+
+    // 1. Register the listener
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_1));
+
+    // 2. Trigger a state change
+    relay_chn_run_forward(ch);
+    vTaskDelay(pdMS_TO_TICKS(test_delay_margin_ms)); // Allow event to be processed
+
+    // 3. Verify the listener was called with correct parameters
+    TEST_ASSERT_EQUAL(1, listener1_info.call_count);
+    TEST_ASSERT_EQUAL(ch, listener1_info.chn_id);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FREE, listener1_info.old_state);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FORWARD, listener1_info.new_state);
+
+    // 4. Unregister to clean up
+    relay_chn_unregister_listener(test_listener_1);
+}
+
+TEST_CASE("Unregistered listener is not called", "[relay_chn][listener]") {
+    uint8_t ch = 0;
+    reset_listener_info(&listener1_info);
+
+    // 1. Register and then immediately unregister the listener
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_1));
+    relay_chn_unregister_listener(test_listener_1);
+
+    // 2. Trigger a state change
+    relay_chn_run_forward(ch);
+    vTaskDelay(pdMS_TO_TICKS(test_delay_margin_ms));
+
+    // 3. Verify the listener was NOT called
+    TEST_ASSERT_EQUAL(0, listener1_info.call_count);
+}
+
+TEST_CASE("Multiple listeners are called on state change", "[relay_chn][listener]") {
+    uint8_t ch = 0;
+    reset_listener_info(&listener1_info);
+    reset_listener_info(&listener2_info);
+
+    // 1. Register two different listeners
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_1));
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_2));
+
+    // 2. Trigger a state change
+    relay_chn_run_forward(ch);
+    vTaskDelay(pdMS_TO_TICKS(test_delay_margin_ms));
+
+    // 3. Verify listener 1 was called correctly
+    TEST_ASSERT_EQUAL(1, listener1_info.call_count);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FREE, listener1_info.old_state);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FORWARD, listener1_info.new_state);
+
+    // 4. Verify listener 2 was also called correctly
+    TEST_ASSERT_EQUAL(1, listener2_info.call_count);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FREE, listener2_info.old_state);
+    TEST_ASSERT_EQUAL(RELAY_CHN_STATE_FORWARD, listener2_info.new_state);
+
+    // 5. Clean up
+    relay_chn_unregister_listener(test_listener_1);
+    relay_chn_unregister_listener(test_listener_2);
+}
+
+TEST_CASE("Listener registration handles invalid arguments and duplicates", "[relay_chn][listener]") {
+    reset_listener_info(&listener1_info);
+
+    // 1. Registering a NULL listener should fail
+    TEST_ASSERT_EQUAL(ESP_ERR_INVALID_ARG, relay_chn_register_listener(NULL));
+
+    // 2. Unregistering a NULL listener should not crash
+    relay_chn_unregister_listener(NULL);
+
+    // 3. Registering the same listener twice should be handled gracefully
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_1));
+    TEST_ESP_OK(relay_chn_register_listener(test_listener_1)); // Second call should be a no-op
+
+    // 4. Trigger a state change and verify the listener is only called ONCE
+    relay_chn_run_forward(0);
+    vTaskDelay(pdMS_TO_TICKS(test_delay_margin_ms));
+    TEST_ASSERT_EQUAL(1, listener1_info.call_count);
+
+    // 5. Clean up
+    relay_chn_unregister_listener(test_listener_1);
+}
+
 
 // ### Tilt Functionality Tests (Conditional)