fix: review fixes for the OTA slot

- The field is hasOtaSlot, like hasEthernet and the other "has" bools.
- The web UI shows the device's reason for any failed upload, not only
  for /restore, so it shows "no OTA slot" from a page that missed the
  header. The update routes' 500 replies give Update.errorString()
  instead of an empty reason.
- Native: the writer leaves hasOtaSlot out when true and writes "os":0.
- e2e: header() next to uptime(); the test skips on firmware without
  the field instead of failing, and matches the reason loosely. The
  README says what the test sends.
- PROTOCOL.md and the comments name only the builds that exist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/README.md

@@ -16,7 +16,7 @@ Runs against a real device on your network, by hand, never in CI. It checks sock
 | `--lockout` | on a Secured device, also fail `/login` until this PC is refused, then wait out the 30 s lockout |
 | `--record` | save hello, the sync and the schema to `test/fixtures/proto2/`, for the web UI and Home Assistant tests |
 
-A plain run sends nothing on air, changes no setting (it saves only values the device already has) and doesn't reboot the device. The command tests send ops that are refused before they reach the radio, and one `setPositions` that sets a shade to the position the sync gave: that drops the fraction of a percent the shade's believed position may have had. The edit tests send edits that are refused, and one shade sort order set to the order it has, which saves the shades again unchanged. The settings tests send saves that are refused, and one that saves the NTP server and time zone the device already has. The restore test sends options that aren't JSON with a file that isn't a backup, so nothing is restored. The frequency scan test pauses the radio's sending for a second or two, as a scan does. It does fill every socket slot for a few seconds, and replays a page load.
+A plain run sends nothing on air, changes no setting (it saves only values the device already has) and doesn't reboot the device. The command tests send ops that are refused before they reach the radio, and one `setPositions` that sets a shade to the position the sync gave: that drops the fraction of a percent the shade's believed position may have had. The edit tests send edits that are refused, and one shade sort order set to the order it has, which saves the shades again unchanged. The settings tests send saves that are refused, and one that saves the NTP server and time zone the device already has. The restore test sends options that aren't JSON with a file that isn't a backup, so nothing is restored. A firmware upload goes only to a device without an OTA slot, which refuses it. The frequency scan test pauses the radio's sending for a second or two, as a scan does. It does fill every socket slot for a few seconds, and replays a page load.
 
 The web UI holds one of the device's 5 socket slots for each open tab, and Home Assistant holds one. The tests that fill every slot fill the ones left, so close the UI's tabs for the full load.
 

test/e2e/test_routes.py

@@ -12,7 +12,7 @@ import urllib.request
 import pytest
 
 from proto_client import ProtoClient, run_sync
-from test_settings import uptime
+from test_settings import header, uptime
 
 # Every route that needs the token under Secured, sent without a body or
 # parameters, so a route that let one through would still change nothing.
@@ -173,18 +173,16 @@ def test_a_socket_from_the_page_the_device_served_is_accepted(host, token):
 
 
 def test_a_firmware_upload_without_an_ota_slot_is_refused(host, token, connect, schema):
-    """Builds with one app slot (C5, C6, debug) say so in the header, and
-    refuse the upload without stopping the radio or rebooting."""
-    c = connect(schema=schema.hash)
-    _, frames, _ = run_sync(c)
-    key = schema.keys["header"]
-    header = next(schema.expand(f["d"][key]) for f in frames if key in f["d"])
-    if header.get("otaSlot", schema.default("otaSlot")):
+    """Builds with one app slot (C6, debug) say so in the header, and refuse
+    the upload without stopping the radio or rebooting."""
+    _, frames, _ = run_sync(connect(schema=schema.hash))
+    # Left out at its default, true, as is a field older firmware doesn't have.
+    if header(schema, frames).get("hasOtaSlot", True):
         pytest.skip("the device has an OTA slot: an upload would start an update")
     uptime_before = uptime(schema, frames)
     status, _, body = upload(host, "/updateFirmware", auth(token))
     assert status == 400, body
-    assert json.loads(body)["desc"] == "This device has no OTA slot, so its firmware is flashed over USB."
+    assert "no OTA slot" in json.loads(body)["desc"]
     # The old reply rebooted half a second later.
     time.sleep(2)
     _, frames, _ = run_sync(connect(schema=schema.hash))

test/e2e/test_settings.py

@@ -86,9 +86,8 @@ def test_a_save_tells_the_other_clients(connect, schema):
 def test_a_wifi_scan_lists_the_networks(connect, schema):
     c = connect(schema=schema.hash)
     _, frames, _ = run_sync(c)
-    header = next(schema.expand(f["d"][schema.keys["header"]]) for f in frames if schema.keys["header"] in f["d"])
     # The connection in use: Ethernet with WiFi as the fallback may be on either.
-    if header.get("connType") != "Wifi":
+    if header(schema, frames).get("connType") != "Wifi":
         pytest.skip("the device isn't on WiFi")
     assert is_ok(schema, op(c, schema, "scanWifi", {}))
     found = c.recv_until(lambda f: f["t"] == schema.type_numbers["wifiNetworks"], timeout=20)[-1]
@@ -152,6 +151,12 @@ def wait_for_restart(host, token, schema, uptime_before):
         time.sleep(2)
 
 
+def header(schema, frames):
+    """The header a sync starts with."""
+    key = schema.keys["header"]
+    return next(schema.expand(f["d"][key]) for f in frames if key in f["d"])
+
+
 def uptime(schema, frames):
     """The uptime, in ms, from the status a sync ends with."""
     key = schema.keys["status"]