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/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))