fix(modem): validate stream type in demuxStream and flush on end

[joshtrichards]

Problem

demuxStream blindly trusts the first byte of each 8-byte header as a valid Docker multiplex stream type. When the stream is not actually multiplexed (e.g. TTY mode) or framing becomes misaligned, a garbage type byte causes a garbage frame length, which misaligns every subsequent frame. The result is corrupted leading bytes in the demuxed output.

This forces downstream consumers to add workarounds like:

// fix dockerode bug returning invalid leading characters
JSON.parse(json.substring(json.indexOf('{')))

See for example:

• nextcloud-libraries/nextcloud-e2e-test-server@780218412e — lib/docker.ts L259-262

Related (may fix) issues:

• Troubles with demuxStream called from docker.run #159 — Troubles with demuxStream called from docker.run
• Add the ability to demux a buffer #129 — Add the ability to demux a buffer

Changes:

• Validate the stream type byte (must be 0, 1, or 2) after reading each header. On an invalid value, flush all buffered data to stdout and switch to raw passthrough for the rest of the stream.
• Listen for the source stream's end event and flush any remaining buffered bytes to stdout instead of silently dropping them.