scanDeviceManually treats any read error as end-of-array, silently truncating the point list

[ravz]

Summary

scanDeviceManually walks the object list by array index and treats any error as end-of-array. A
transient APDU timeout partway through therefore truncates the point list silently, and the truncated
list is then stored as if it were complete.

Found while investigating #49. Not the cause of that issue, but it directly undermines the fallback
path that #49's fix depends on.

Detail

bacnet_client.js:1288-1333:

that.client.readProperty(addressObject, {type: DEVICE, instance: deviceId}, OBJECT_LIST, readOptions,
  (err, value) => {
    if (err) {
      resolve(discoveredPointList);   // any error == "we're done"
    }
    if (value) {
      discoveredPointList.push(value.values[0]);
      index++;
      send(index);
    }
  }
);

The scan starts at index 1 and never reads index 0 (the element count), so it has no idea how many
objects it is supposed to find. The only termination signal is an error — which is indistinguishable
from a dropped packet, a busy device, or a router hiccup.

Consequences:

• A timeout on object 12 of 48 yields 11 objects, resolved as success.
• The caller cannot tell truncation from completion. getDevicePointListWithoutObjectList
  (bacnet_client.js:1267) calls setPointsList(result) and setLastSeen(Date.now()) either way.
• A failure on the first index resolves []. setPointsList([]) iterates nothing and leaves the
  existing [device] entry intact, so total failure looks identical to "no new points".

There is also no err/value else-branch: if a callback ever fires with neither, the promise never
settles.

Proposed fix

Read array index 0 first to get the element count, then read 1..count, and resolve only when
index > count. Anything short of count is a failure and should reject, not resolve.

Devices that reject the index-0 read can keep the current walk-until-error behaviour as a degraded
mode, but it should be logged as such rather than presented as a complete list.

Make the err branch return after resolving, and add an else-branch so a callback with neither
err nor value settles the promise instead of hanging.

Acceptance criteria

1. A device that errors partway through the index scan produces a rejection or an explicitly-flagged
   partial result, not a silently truncated point list stored as complete.
2. A device that answers index 0 has exactly count objects read, and the result is only accepted when
   all count are present.
3. A callback that fires with neither err nor value settles the promise.