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

[gijzelaerr]

@bvanelli I refreshed this branch onto current master, added coverage for the substreamed fallback, and requested your review.

Since you have both current V2 and legacy/session-key PLC coverage, could you also try one db_write_multi() round trip on each available protocol path? The useful result is simply the PLC model/protocol, whether all values were written, and whether a following read returns them; no raw capture is needed unless it fails.

pip install --upgrade "python-snap7 @ git+https://github.com/gijzelaerr/python-snap7.git@feat/795-s7commplus-multi-write"

The current head passes 1,826 tests, the complete pre-commit suite, and the package build.