feat(L2CAP): add disconnect API and harden CoC send/error handling

- Add NimBLEL2CAPChannel::disconnect() and getConnHandle().
- Fix CoC TX mbuf ownership handling in writeFragment():
  treat BLE_HS_ENOMEM / BLE_HS_EAGAIN as consumed buffer,
  only free local tx mbuf on BLE_HS_EBUSY.

Author: mickeyl

src/NimBLEL2CAPChannel.cpp

@@ -134,8 +134,14 @@ int NimBLEL2CAPChannel::writeFragment(std::vector<uint8_t>::const_iterator begin
 
             case BLE_HS_ENOMEM:
             case BLE_HS_EAGAIN:
+                /* ble_l2cap_send already consumed and freed txd on these errors */
+                NIMBLE_LOGD(LOG_TAG, "ble_l2cap_send returned %d (consumed buffer). Retrying shortly...", res);
+                ble_npl_time_delay(ble_npl_time_ms_to_ticks32(RetryTimeout));
+                continue;
+
             case BLE_HS_EBUSY:
-                NIMBLE_LOGD(LOG_TAG, "ble_l2cap_send returned %d. Retrying shortly...", res);
+                /* Channel busy; txd not consumed */
+                NIMBLE_LOGD(LOG_TAG, "ble_l2cap_send returned %d (busy). Retrying shortly...", res);
                 os_mbuf_free_chain(txd);
                 ble_npl_time_delay(ble_npl_time_ms_to_ticks32(RetryTimeout));
                 continue;
@@ -197,6 +203,28 @@ bool NimBLEL2CAPChannel::write(const std::vector<uint8_t>& bytes) {
     return true;
 }
 
+bool NimBLEL2CAPChannel::disconnect() {
+    if (!this->channel) {
+        NIMBLE_LOGW(LOG_TAG, "L2CAP Channel not open");
+        return false;
+    }
+
+    int rc = ble_l2cap_disconnect(this->channel);
+    if (rc != 0 && rc != BLE_HS_ENOTCONN && rc != BLE_HS_EALREADY) {
+        NIMBLE_LOGE(LOG_TAG, "ble_l2cap_disconnect failed: rc=%d %s", rc, NimBLEUtils::returnCodeToString(rc));
+        return false;
+    }
+
+    return true;
+}
+
+uint16_t NimBLEL2CAPChannel::getConnHandle() const {
+    if (!this->channel) {
+        return BLE_HS_CONN_HANDLE_NONE;
+    }
+    return ble_l2cap_get_conn_handle(this->channel);
+}
+
 // private
 int NimBLEL2CAPChannel::handleConnectionEvent(struct ble_l2cap_event* event) {
     channel = event->connect.chan;

src/NimBLEL2CAPChannel.h

@@ -56,6 +56,14 @@ class NimBLEL2CAPChannel {
     /// NOTE: This function will block until the data has been sent or an error occurred.
     bool write(const std::vector<uint8_t>& bytes);
 
+    /// @brief Disconnect this L2CAP channel.
+    /// @return true on success, false on failure.
+    bool disconnect();
+
+    /// @brief Get the connection handle associated with this channel.
+    /// @return Connection handle, or BLE_HS_CONN_HANDLE_NONE if not connected.
+    uint16_t getConnHandle() const;
+
     /// @return True, if the channel is connected. False, otherwise.
     bool isConnected() const { return !!channel; }