Update bundled pip binary to v0.3.50

The pip launcher prefers the binary bundled in the wheel over the release
download, so the committed 0.3.49 binary masked every code fix. Ship the
0.3.50 build (model persistence, mode arrows, ask_user, cloud reply fixes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: metiu1