Add round-trip regression tests for _parse_ccsds_head

[namurphy]

Adds tests/decommutator/test_ccsds_reader_pipeline.py with unit tests for _parse_ccsds_head. These are the first tests of an individual decommutator function; until now the decommutator was only tested end to end through the command line tool.

A test helper, _make_ccsds_head, packs header fields into 10 bytes with struct.pack, independently of the code under test. The tests check that:

• Decoding an encoded header gives back the original fields. Cases cover a typical SPC header, a few APIDs including the largest (0x7FF), all fields zero, and all fields at their maximum.
• Setting every bit of one field does not change any other field.
• Bytes after the first 10 are ignored.
• Headers shorter than 10 bytes raise ValueError.

🤖 Generated with Claude Code

[namurphy]

I read through these and the tests seemed sensible, though admittedly I don't have a full understanding of the functionality.

Important

These tests should be thought of as regression tests to make sure that changes to the code preserve existing behavior.