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 regular-session writes in one existing SET_MULTI_VARIABLES request
• preserve V1-initial/substreamed compatibility by issuing one SET_VAR_SUBSTREAMED request per item
• cover disconnected behavior, cross-DB sync/async integration, and the substreamed fallback

Verification

• uv run --frozen pytest (1826 passed, 78 skipped)
• uv run --frozen pre-commit run --all-files
• uv build --no-sources

Fixes #795