CCSDSPacketBytes accepts inputs shorter than a primary header, then header properties raise bare IndexError

Context

CCSDSPacketBytes (in space_packet_parser/generators/ccsds.py) is a bytes subclass with properties that read the six-byte CCSDS primary header (version_number, type, secondary_header_flag, apid, sequence_flags, sequence_count, data_length, header_values). Nothing checks the length at construction, so an instance shorter than HEADER_LENGTH_BYTES is created without complaint and every header property then fails with an uninformative error:

>>> from space_packet_parser.generators.ccsds import CCSDSPacketBytes
>>> CCSDSPacketBytes(b"\x07").apid
IndexError: index out of range

UDPPacketBytes in generators/udp.py handles the same situation differently. Its __new__ rejects anything shorter than its 8-byte header with ValueError("UDP packet must be at least 8 bytes (got 1 bytes)"). The two bytes subclasses now disagree on how to treat a short input.

#282 made CCSDSPacketBytes.__str__ safe for short input so it can be used in diagnostic messages, but deliberately left construction and the header properties alone because changing them is a compatibility question rather than an error-path fix.

Driving Requirements

A caller who constructs a CCSDSPacketBytes that cannot hold a primary header should get a clear error naming the problem, at a predictable point, consistent with UDPPacketBytes.

Recommended Approach

Validate at construction, matching UDPPacketBytes:

• Add a __new__ to CCSDSPacketBytes that raises ValueError when len(value) < HEADER_LENGTH_BYTES, with a message stating the minimum and actual lengths.
• Document the ValueError in the class docstring.
• Confirm ccsds_generator never constructs a short CCSDSPacketBytes on its truncated-final-packet path, or adjust it so the existing "not enough bytes" handling still produces the current warning rather than a new ValueError.
• Add unit tests in tests/unit/test_generators/test_ccsds.py for the rejection and for the boundary (exactly six bytes accepted).
• Once construction guarantees a full header, the short-input branch added to __str__ in #282 becomes unreachable and can be removed along with its test.

This is a behavior change for any caller that currently builds short CCSDSPacketBytes instances, so it should be called out in the changelog and land in a minor or major release per the project's semantic versioning practice, not a patch.

Alternative Considered

Keep construction permissive and have each header property raise ValueError with a message instead of IndexError. This avoids the compatibility change but leaves the two bytes subclasses inconsistent and spreads the length check across seven properties.

Related

• #276 / #282: made __str__ safe for short input on the error-reporting path.

Start in space_packet_parser/generators/ccsds.py and compare CCSDSPacketBytes construction with UDPPacketBytes in generators/udp.py. Run tests/unit/test_generators/test_ccsds.py, including the truncated-final-packet path, rejection of short inputs, and the six-byte boundary case. Done means short construction has a clear documented error, valid boundary input works, existing warning behavior remains, and the changelog records the compatibility change.