zephyr-cp/wifi: subscribe to NET_EVENT_WIFI_SCAN_RESULT

[mikeysklar]

Wi-Fi scanning on zephyr-cp returns zero networks on every board.

_event_handler in ports/zephyr-cp/common-hal/wifi/__init__.c has a NET_EVENT_WIFI_SCAN_RESULT case that queues each AP as it arrives, but that event is not in the mask passed to net_mgmt_init_event_callback. The case has never run, so nothing is ever queued and every scan comes back empty.

NET_EVENT_WIFI_RAW_SCAN_RESULT is in the mask, which is probably how this went unnoticed, but it is not a substitute. It carries raw beacon frames and only fires when CONFIG_WIFI_MGMT_RAW_SCAN_RESULTS is enabled, which it is not by default.

Testing

Raspberry Pi Pico 2 W, in-tree board raspberrypi_rpi_pico2_w_zephyr, built from 069144c on Linux and flashed over SWD with pyOCD via a CMSIS-DAP Debug Probe.

import wifi
len([1 for n in wifi.radio.start_scanning_networks()])

build	result
main	0
main + this patch	204

Same board, same probe, same script. The before run is this commit reverted and rebuilt, nothing else changed. Both returned promptly to the prompt.

Anyone with a Pico 2 W can reproduce this, no vendor hardware needed.

Notes

This is the root cause behind #11214, which I opened earlier and have closed. That one bounded the scan wait instead of fixing why no results arrive, and @tannewt was right to push back on it.

AI assistance was used to draft this change. I ran the hardware testing above myself and confirmed the before and after numbers on my own board.

[tannewt]

One request to remove a PR-specific comment

[tannewt]

No need for this. The addition is self explanatory.

Suggested change

	// SCAN_RESULT delivers the parsed per-AP entries. Without it the
	// handler's NET_EVENT_WIFI_SCAN_RESULT case never runs and scans
	// always return zero networks. RAW_SCAN_RESULT is not a substitute:
	// it carries raw beacon frames and only fires when
	// CONFIG_WIFI_MGMT_RAW_SCAN_RESULTS is enabled, which it is not by
	// default.