fix(thp): decode a packet that exactly fills the packet size

[munzzyy]

TrezorDecoder.decodePackage decides whether a THP v2 packet already holds the whole message by comparing the length field against the bytes left in the reader:

if (length >= reader.remainingLength) {
  final payload = reader.read(reader.remainingLength);
  return TrezorPackageV2(headers: headers, payload: payload);
}

final payload = reader.read(length - crcLength);
final crc = reader.readUint32();

length counts the payload plus the 4 byte CRC, and the second branch reads exactly that many bytes, so the message is complete whenever remainingLength >= length. With >= on the incomplete side the equality case lands in the wrong branch. That case is the largest message that still fits in a single packet: 235 payload bytes over a 244 byte BLE MTU, or 55 payload bytes over a 64 byte USB packet. It comes back with crc == null, so needsContinuationPacket is true and both transports wait for a packet the device is never going to send. _readResponse in trezor_usb_manager.dart keeps calling transferIn(), and the notify handler in trezor_gatt_gateway.dart returns early without completing the request.

The change is the comparison. I also added test/trezor/protocol/decoder_test.dart with a helper that builds a real THP packet (control byte, channel, length, payload, CRC32, padded the way the device pads) and covers three cases: a short padded packet, the exact fit packet, and a message too large for one packet.

Running the new test on trunk with the one line change reverted:

00:00 +0: TrezorDecoder decodePackage decodes a padded packet
00:00 +1: TrezorDecoder decodePackage decodes a packet that exactly fills the packet size
00:00 +1 -1: TrezorDecoder decodePackage decodes a packet that exactly fills the packet size [E]
  Expected: <235>
    Actual: <239>

  package:matcher                              expect
  test/trezor/protocol/decoder_test.dart 52:7  main.<fn>.<fn>

00:00 +1 -1: TrezorDecoder decodePackage asks for a continuation packet when the message does not fit
00:00 +2 -1: Some tests failed.

239 is the whole remainder including the CRC. With the fix:

00:00 +0: TrezorDecoder decodePackage decodes a padded packet
00:00 +1: TrezorDecoder decodePackage decodes a packet that exactly fills the packet size
00:00 +2: TrezorDecoder decodePackage asks for a continuation packet when the message does not fit
00:00 +3: All tests passed!

flutter test for the whole package is green at 31 tests, and flutter analyze reports nothing on either changed file. Flutter 3.44.6, Dart 3.12.2, Linux.