Pin pylance below 8 and disable blob compaction

[FANNG1]

Summary

• Pin pylance below 8.0.0 so lance-ray 0.4.2 distributed vector index creation keeps working
• Disable Stage 5 compaction for now and keep delete + cleanup behavior
• Update workflow tests and README to document that blob v2 compaction should be re-enabled after moving to pylance 8.x with a compatible lance-ray release

Validation

• uv run pytest tests/workflow/test_index_manage.py tests/image/test_embedding_pipeline.py -ra
• Verified versions: pylance 7.0.0, lance-ray 0.4.2, lance-namespace 0.7.7
• Verified minimal lance_ray.create_index IVF_FLAT build succeeds with pylance 7.0.0