fix: correct CCSDS packet length calculation

[0rlych1kk4]

Summary

Fix CcsdsTcPacketOwned::len_written() and
CcsdsTmPacketOwned::len_written() to use the length of the already-serialized
payload bytes instead of the postcard serialized size of the Vec<u8> container.

Previously, len_written() overestimated the actual packet length because
postcard serialization of a Vec<u8> includes its length prefix, while
write_to_bytes() copies the payload bytes directly.

Testing

Added regression tests verifying:

packet.len_written() == packet.to_vec().len()

For both TC and TM packets.

Verified with:

cargo test -p types

[robamu]

thanks for finding this and fixing it!

[0rlych1kk4]

Thanks, glad I spotted it. Nice to get the length calculation and regression coverage cleaned up.