extmod/bluetooth: Add gap_unpair() for all BLE backends.

[andrewleech]

Summary

There's no way to programmatically remove bonding keys in the current BLE API. Tests that pair/bond need to clear stale keys on startup to avoid failures from leftover state, and applications that manage multiple bonds need per-peer removal.

This adds BLE.gap_unpair() with two forms: no-arg clears all bonds, two-arg (addr_type, addr) removes a specific peer. Implemented for NimBLE (ble_gap_terminate + ble_store_util_delete_peer) and BTstack (gap_delete_bonding). The Zephyr backend already has a dormant implementation that activates when the Zephyr BLE integration lands.

Testing

• ble_gap_unpair.py test added: connects, pairs with bonding, disconnects, then calls per-address gap_unpair() on both sides.
• gap_unpair() no-arg calls added to ble_gap_pair.py, ble_gap_pair_bond.py, and ble_subscribe.py startup to clear stale bonds.
• Tested on PYBD-SF6W (NimBLE).

Generative AI

I used generative AI tools when creating this PR, but a human has checked the code and is responsible for the description above.

[github-actions]

Code size report:

Reference:  rp2/modules/rp2.py: Don't corrupt globals on asm_pio() exception. [c3ca843]
Comparison: extmod/bluetooth: Add gap_unpair() for all BLE backends. [merge of a32cee1]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
      esp32:  +240 +0.014% ESP32_GENERIC[incl +32(data)]
     mimxrt:    +0 +0.000% TEENSY40
        rp2:   +72 +0.008% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32