Bluetooth test robustness: reuse lib helpers and harden scan/firmware checks

[smuppand]

This PR refines the Bluetooth validation flow and consolidates common logic into shared helpers to make CI results more meaningful and less flaky.

Key changes

• BT_ON_OFF:

  • Use bt_ensure_controller_visible and public-addr/bootstrap logic instead of inline controller checks.
  • Honour optional BT_ADAPTER env (from LAVA/CLI); fall back to findhcisysfs when not provided.
  • Ensure bluetoothd presence and adapter handling follows common pattern with clear SKIP/FAIL paths.

• BT_SCAN:

  • Switch to the non-expect scan flow using bt_set_scan + bt_get_discovering and controller-visibility helpers.
  • Support BT_SCAN_TARGET_MAC / BT_TARGET_MAC to validate a specific device when provided, else require at least one device in bluetoothctl devices.
  • Poll Discovering state after scan ON/OFF and fail when no devices are discovered, improving CI signal.
  • Keep all Bluetooth-specific logic in lib_bluetooth.sh, leaving run.sh mostly as orchestration.

• BT_FW_KMD:

  • Tighten btfwloaded() to scan dmesg for QCA/firmware issues (e.g. tx timeout/version-read failures).
  • Use shared helpers for controller visibility and basic sanity checks instead of ad-hoc parsing.

• lib_bluetooth.sh:

  • Add reusable helpers: bt_ensure_controller_visible, bt_set_scan, bt_get_discovering, and controller MAC parsing.
  • Centralise public-addr/bootstrap flow and clean up bluetoothctl output handling.
  • Keep function names and existing call patterns intact where previously used.

• functestlib.sh:

  • Trim Bluetooth-specific bits now moved into lib_bluetooth.sh, keeping this file focused on generic logging, dependency checks, and result handling.
  • No changes to .res file semantics or common test harness behaviour.

[bhargav0610]

Looking good for me

[abbajaj806]

LGTM