MAVLink STATUSTEXT chunking mishandles ID zero and exact 50-byte boundaries #29251

Bug description

The PX4 MAVLink STATUSTEXT sender can produce chunked messages that do not follow the MAVLink STATUSTEXT chunking contract. The relevant code is src/modules/mavlink/streams/STATUSTEXT.hpp.

The MAVLink definition says id == 0 means the message is a single chunk, and a NUL anywhere in the 50-byte text field marks the last chunk.

Problems

1. The first status text uses ID zero. _id is initialized to zero, and the sender assigns msg.id = _id++. If the first published status text is longer than 50 bytes, every chunk has ID zero. A receiver following the spec therefore treats each chunk as an independent message rather than assembling them. The same issue recurs when the 16-bit ID wraps to zero.

2. An exact multiple of 50 bytes has no terminating chunk. The sender copies full 50-byte chunks. When the final remainder is exactly 50 bytes, it sends that full chunk and breaks because text_size <= max_chunk_size; it does not send a following NUL-terminated chunk. Receivers cannot tell that the sequence is complete and may eventually time out or report a missing chunk. For example, a 100-byte message is sent as chunks 0 and 1, both full-width, with no NUL in either chunk.

Expected behavior

• Chunked messages use a nonzero id for every chunk, including across ID wraparound.
• The final chunk contains a NUL in the text field. For a message whose length is an exact multiple of 50 bytes, send an empty NUL-terminated chunk after the full chunks (or otherwise ensure the last transmitted chunk contains a NUL without dropping message bytes).

How to reproduce

Exercise the STATUSTEXT sender with a message longer than 50 bytes as the first status text, and with text lengths of exactly 50, 100, or 150 bytes. Inspect id, chunk_seq, and the raw 50-byte text field in the transmitted packets.

I checked this against the MAVLink STATUSTEXT definition in the PX4 tree and the local PX4 sender implementation.