zephyr-cp: Improve BLE error reporting: raise GATT ATT errors as specific exceptions

[dhalbert]

• Follow-up to espressif: report GATT ATT errors as specific exceptions #11550 and nordic: report GATT write errors and map more ATT errors #11553

Diagnosis and code by Claude, reviewed by @dhalbert.

Error reporting

• Before this PR, BLE GATT client reads and writes just raised ATT errors as OSError(-N), where N was the ATT error code.
• A new check_gattc_status() function now maps the same ATT errors as espressif: report GATT ATT errors as specific exceptions #11550 and nordic: report GATT write errors and map more ATT errors #11553 into readable exception messages:

ATT error	Exception
0x02 Read Not Permitted	BluetoothError: read not permitted
0x03 Write Not Permitted	BluetoothError: write not permitted
0x05 Insufficient Authentication	SecurityError: Insufficient authentication
0x08 Insufficient Authorization	SecurityError: Insufficient authorization
0x0C Insufficient Encryption Key Size, 0x0F Insufficient Encryption	SecurityError: Insufficient encryption
0x0D Invalid Attribute Value Length	ValueError: Invalid length
any other	BluetoothError: Unknown gatt error: 0x00xx

• Also, Zephyr reports a disconnect during the request, and other local failures, as BT_ATT_ERR_UNLIKELY (0x0E). So check_gattc_status() checks the connection first and raises ConnectionError if it dropped.
  • Each request holds a reference on the bt_conn until that check, so it is safe after a disconnect.

Testing

nRF54LM20 DK with this PR as central, Feather nRF52840 on nordic with #11553 as peripheral:

Test	Result
read/write NO_ACCESS, unpaired read of ENCRYPT_NO_MITM, over-length write to fixed_length	read not permitted, write not permitted, Insufficient authentication, Invalid length
Ordinary read and write	unchanged
Disconnect during a read/write loop, then reconnect and disconnect again	ConnectionError, then no problems
ble_uart_echo_client.py from the BLE library examples	echoes received, no errors
pytest tests/bsim -k nrf5340	61 passed