chore(examples): update shebang from poetry to uv (#1497)

The project migrated from poetry to uv, and CONTRIBUTING.md documents
uv as the package manager. Newer examples (agents.py) already use
`#!/usr/bin/env -S uv run python`. Update the 4 remaining examples
that still referenced poetry.

Co-authored-by: Junhyuk Lee <xodn348@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>