Use designated initializer for NimBLEScan::m_scanParams and guard disable_observer_mode field for ESP-IDF >= 5.4.2

The NimBLEScan constructor previously used positional struct initialization,
which no longer matches the ble_gap_disc_params layout in newer ESP-IDF
versions (>= 5.4.2) where the field `disable_observer_mode` was added.

This is caused by -Wmissing-field-initializers.

Switch to designated initializers to make the field assignments explicit
and more robust across ESP-IDF/NimBLE revisions. The new field is only
initialized when building against ESP-IDF 5.4.2 or later to maintain
backwards compatibility.

Author: stdstijn

src/NimBLEScan.cpp

@@ -32,10 +32,20 @@ static NimBLEScanCallbacks defaultScanCallbacks;
  */
 NimBLEScan::NimBLEScan()
     : m_pScanCallbacks{&defaultScanCallbacks},
-      // default interval + window, no whitelist scan filter,not limited scan, no scan response, filter_duplicates
-      m_scanParams{0, 0, BLE_HCI_SCAN_FILT_NO_WL, 0, 1, 1},
+      m_scanParams{
+          .itvl              = 0,                       // default interval
+          .window            = 0,                       // default window
+          .filter_policy     = BLE_HCI_SCAN_FILT_NO_WL, // no whitelist scan filter
+          .limited           = 0,                       // no limited scan
+          .passive           = 1,                       // no scan response
+          .filter_duplicates = 1,                       // filter duplicates
+# if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(5, 4, 2)
+          .disable_observer_mode = 0, // observer role enabled
+# endif
+      },
       m_pTaskData{nullptr},
-      m_maxResults{0xFF} {}
+      m_maxResults{0xFF} {
+}
 
 /**
  * @brief Scan destructor, release any allocated resources.