Run fails if input buffer is only partially read

It is possible to write a codespec that only partially reads from an input buffer, causing the run to fail, even if the job returns exit code 0.

Example: Copy stdin to stdout up to a maximum number of bytes, then exit successfully.

It is not required that a codespec read its input buffers. However, once the codespec begins reading from stdin or a named input pipe, should it be required to read that stream until EOF? If so, we'll document it. If not, then perhaps we need to change the logic to allow partial reads, and respect the return code of the main container.

Start by reproducing the described codespec that copies only a bounded amount from stdin or a named input pipe and exits with code 0. Trace the run handling for partially read input, then determine whether the completed behavior should document an EOF requirement or permit partial reads while honoring the main container's return code.