…gth (#16852)

Motivation:

`MqttDecoder.decodePayload`'s `default` branch returns `null` for
payload-less message types (`PINGREQ`, `PINGRESP`, `DISCONNECT`, `AUTH`,
`CONNACK`, `PUBACK`, `PUBREC`, `PUBREL`, `PUBCOMP`) without checking
that `bytesRemainingInVariablePart` is `0`. A fixed header that claims a
non-zero Remaining Length for one of these types is silently accepted,
and the leftover bytes are interpreted as another packet.

The reporter's exact repro from #16851 is `C0 02 D0 00`: a `PINGREQ`
with Remaining Length `2` (invalid per [MQTT
3.1.1](https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html)
and [MQTT
5.0](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html),
which both require Remaining Length `0` for `PINGREQ`) is currently
decoded as a valid `PINGREQ` followed by a valid `PINGRESP`.

Modification:

Call `validateNoBytesRemain(0)` at the start of the `default` branch.
The helper — already used by the `SUBSCRIBE` and `UNSUBSCRIBE` payload
decoders — subtracts `0` from `bytesRemainingInVariablePart` and throws
`DecoderException` when the result is non-zero, surfacing a single
invalid message instead of letting leftover bytes leak into the next
decode pass.

Added `MqttCodecTest#testPingReqWithNonZeroRemainingLengthIsRejected`
that feeds the reporter's exact byte sequence and verifies a single
failed message is produced. Confirmed it fails on the unfixed code
(`expected: <true> but was: <false>`) and passes with the fix; all 112
`codec-mqtt` tests pass locally.

Result:

Malformed payload-less MQTT frames are flagged as a single invalid
message rather than allowed to silently spill into the next packet. The
fix applies uniformly to every message type that goes through the
`default` branch.

Fixes #16851.

---------

Co-authored-by: Norman Maurer <norman_maurer@apple.com>
Co-authored-by: Chris Vest <christianvest_hansen@apple.com>
(cherry picked from commit d89cf88)