fix(server): notify characteristics on client disconnect

[m-mcgowan]

Summary

When a BLE client disconnects, characteristics with active subscriptions are not notified. This leaves applications with stale subscription state, causing issues with reconnection.

This PR adds code to the BLE_GAP_EVENT_DISCONNECT handler to iterate through all characteristics and call processSubRequest(peerInfo, 0) for any that had active subscriptions from the disconnecting client.

Problem

Applications using wrappers like NuS-NimBLE-Serial track subscriber counts via the onSubscribe callback. Without this fix:

1. Client connects and subscribes → onSubscribe(subValue=1) called → count = 1
2. Client disconnects abruptly → no callback → count stays at 1
3. Client reconnects → subscription state is stale → reconnection fails

Solution

In BLE_GAP_EVENT_DISCONNECT, before calling onDisconnect:

• Iterate through all services and characteristics
• For each characteristic with an active subscription from the disconnecting client
• Call processSubRequest(peerInfo, 0) to trigger the onSubscribe callback with subValue=0

This ensures:

1. The onSubscribe callback is called with subValue=0
2. Applications tracking subscriber counts get properly notified
3. Subscription state is cleaned up for reliable reconnection

Testing

Tested with ESP32-S3 using the ESP-IDF framework (with Arduino as a component) and NuS-NimBLE-Serial. Before the fix, reconnection after disconnect would fail. After the fix, reconnection works reliably.

Note

An equivalent fix has been submitted to NimBLE-Arduino (#1081 in h2zero/NimBLE-Arduino) for Arduino-only builds.