feat(s7commplus): add multi-write APIs

[gijzelaerr]

Summary

• add db_write_multi() to the synchronous and asynchronous S7CommPlus clients
• add the requested write_multi() convenience alias
• encode all writes in one existing SET_MULTI_VARIABLES request
• cover disconnected behavior and cross-DB sync/async integration

Verification

• uv run --frozen --extra test --extra s7commplus pytest -q (1641 passed, 82 skipped)
• uv run --frozen pre-commit run --all-files
• uv build --no-sources

Fixes #795