Fix compilation and runtime crashes on ESP32 core 3.x

[kevinagomez]

Problem

ESP32-BLE-Keyboard fails to compile and crashes at runtime on
ESP32 Arduino core 3.x (tested on XIAO ESP32-C6 with Seeed core 3.3.7).

Symptoms

• Compilation error: cannot convert std::string to String
• Runtime: Guru Meditation Load access fault (MCAUSE 0x05, MTVAL 0x0000002c)

Root Causes Fixed

1. std::string → Arduino String throughout constructor, members, signatures
2. substr() → substring() for Arduino String API
3. BLEDevice::init() and setValue() — pass String directly
4. onConnect/onDisconnect — null-check BLE2902 descriptors before use
   (core 3.x returns nullptr from getDescriptorByUUID — this was the
   primary crash cause)
5. onWrite() — fix dangling pointer from temporary String destruction

Tested On

• Hardware: Seeed XIAO ESP32-C6
• Core: Seeed esp32 3.3.7
• Sketch: BLE HID keyboard sending keystrokes successfully