vNext: Accept a scan reply without cid

[Twintop]

Context

This is a bug fix in preparation for Gree zone controller support. I found it while porting the changes I made for my own zone controller (Gree LE60-13/GH) to this project. The zone controller support itself will come in a later PR. Background and the full plan: https://github.com/Twintop/ha-gree-zone-controller/blob/main/upstream/README.md

Symptom

A Gree LE60-13/GH zone controller (WiFi module ME31-00/C13, firmware 362001062617+U-W05SAV1.27.bin) cannot be discovered at all. Discovery of its IP fails with:

pydantic_core._pydantic_core.ValidationError: 1 validation error for DeviceScanInfoResponse
cid
  Field required [type=missing, input_value={'t': 'dev', ...}, input_type=dict]

Cause

The unit answers the scan with a dev pack that has no cid:

{"t":"dev","bc":"gree","catalog":"gree","mid":"50","lock":0,"model":"gree","name":"GR-ZCntrlr_5000_02_ffee_EC","series":"gree","vender":"2","ver":"V1.2.1","brand":"gree","mac":"9424b8c0ffee","subCnt":5}

DeviceScanInfoResponse requires cid, so _process_local_scan_response() raises before anything else runs. Nothing in the integration reads cid.

Change

• cid is optional in DeviceScanInfoResponse.
• New test test_a_scan_reply_without_cid_is_accepted, with a fake device whose scan reply has no cid. It fails with the error above without this change.
• One line in docs/protocol.md.

Testing

• pytest: 255 passed (the 254 before, plus the new test).
• Ruff passes. Mypy and Pylint show nothing new compared with 5.0-dev. I ran them without Home Assistant installed, so I compared the findings with the base branch.
• Real device, LE60-13/GH zone controller: before the change, discovery of its IP raised the error above. With the change, discovery returns the 5 sub-units behind it.

[RobHofmann]

Thanks, this is a clean fix.

• Confirmed nothing reads cid from the scan reply; _process_local_scan_response only uses mac, name and the optional fields.
• The new test fails on 5.0-dev with the exact ValidationError and passes with the change. Full suite: 255 passed.
• Ran it on my dev instance against my own unit (which does send cid): binds and polls normally, no errors. I could not test the zone controller path myself.

The Ruff failure is unrelated: CI picked up Ruff 0.17.0 today, which flags the existing import in const.py. The last 5.0-dev run used 0.16.10 and passed. I will fix that separately.