fix(s7commplus): renew legacy session keys

[gijzelaerr]

Closes gijzelaerr/s7commplus#4

Summary

• renew legacy SessionKeys every 25 minutes by default, with a configurable/disableable interval
• serialize the challenge and SecurityKey exchange with application requests
• keep the old key active through response-HMAC verification, then switch atomically
• close the connection and surface a stable error when renewal fails
• cancel renewal activity during disconnect and document the lifecycle

Testing

• pre-commit run --all-files
• uv run pytest -q --tb=short (2104 passed, 82 skipped)
• uv build --quiet

Dependency

This is stacked on #884 because renewal must verify the PLC response with the old SessionKey before installing the new key.

[gijzelaerr]

This S7CommPlus work is now tracked in gijzelaerr/s7commplus#19. Pull requests cannot be transferred between repositories, so this PR is being closed in favor of the new-repository tracking issue.