[BLE] Add BM6 Battery Monitor connection support (#2274)

* [BLE] Add BM6 Battery Monitor connection support

Implement BLE connection support for BM6 Battery Monitor devices,
following the BM2 pattern with BM6-specific protocol handling.

Key features:
- Encrypted command/response using AES-128 CBC
- Read voltage, temperature, and State of Charge (SoC)
- Home Assistant MQTT Discovery support
- Device tracker integration

Technical implementation:
- Add BM6_connect class with encrypted command handling
- Encryption key (static const): "leagend\xff\xfe0100009"
- Parse hex string positions for voltage (15-17), temp (8-9), SoC (12-13)
- Add discovery with 3 sensors (voltage, temperature, battery %)
- Validate message signature (D15507)
- Add canWrite() check before writing to characteristic
- Empty stub for non-ESP32 builds

Code quality improvements:
- Extract AES key as static constant to avoid duplication
- Use shared jsonVoltBM template for both BM2 and BM6
- Clean up commented-out reasoning in parsing logic

Based on reverse engineering from:
https://github.com/JeffWDH/bm6-battery-monitor

Files modified:
- main/gatewayBLEConnect.h: Add BM6_connect class
- main/gatewayBLEConnect.cpp: Implement BM6 connection logic
- main/gatewayBT.cpp: Add detection, connection, and discovery handlers
- main/config_mqttDiscovery.h: Rename jsonVoltBM2 to jsonVoltBM

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* Add BM6 to exceptions

---------

Co-authored-by: Florian <1technophile@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

main/gatewayBLEConnect.h

@@ -48,6 +48,14 @@ public:
   void publishData() override;
 };
 
+class BM6_connect : public zBLEConnect {
+  void notifyCB(NimBLERemoteCharacteristic* pChar, uint8_t* pData, size_t length, bool isNotify);
+
+public:
+  BM6_connect(NimBLEAddress& addr) : zBLEConnect(addr) {}
+  void publishData() override;
+};
+
 class GENERIC_connect : public zBLEConnect {
   std::vector<uint8_t> m_data;