fix: narrow write_multi_vars item types

[gijzelaerr]

Summary

• replace the redundant list cast with per-item type narrowing
• reject mixed S7DataItem and dictionary lists with a clear TypeError
• keep compatibility with both the pinned mypy hook and newer mypy releases

Validation

• uv run --frozen pre-commit run --all-files
• pytest -q (1821 passed, 78 skipped)
• mypy snap7/client.py
• uv build --no-sources