fix(examples): correct geo3k VLM default env

[zhangdw156]

Summary

• Correct the geo3k VLM multi-turn default env module path to the existing example package.
• Update the module execution comment to match the actual package name.
• Add a lightweight static regression test so the default env module keeps pointing to an existing file.

Tests

• uvx --from pytest pytest tests/test_geo3k_vlm_multi_turn_paths.py
• uvx ruff check examples/geo3k_vlm_multi_turn/rollout.py tests/test_geo3k_vlm_multi_turn_paths.py
• git diff --check