fix: handle examples without requirements in setup

[saurabhhhcodes]

Summary

• copies the selected example before dependency installation so Docker builds do not fail when an example has no requirements.txt
• installs Python dependencies only when requirements.txt exists
• makes setup.sh source either .venv/bin/activate or .venv/Scripts/activate for Unix and Git Bash/MSYS virtualenv layouts
• updates the printed activation hint to match the detected activation script

Verification

• bash -n setup.sh
• git diff --check
• docker --version (not available in this local environment, so I could not run a Docker build here)

Fixes #143

[gautammanak1]

Thanks for the contribution 🚀

The CI for this PR is currently failing. Please fix the following so we can move forward:

• changelog-check — add an entry to contributors/CHANGELOG.md (community agents) or root CHANGELOG.md
• stargazer-gate — please ⭐ star this repository (the gate passes once you've starred)

Once the checks are green it'll be ready for maintainer review. Let me know if you need any help! 🙌