fix(s7commplus): probe family-only session keys

[gijzelaerr]

Closes gijzelaerr/s7commplus#3

Some legacy PLCs advertise only a SessionKey family such as 00, 01, or 03. This adds a bounded fallback that:

• keeps complete-fingerprint selection unchanged
• enumerates only bundled keys from the reported family
• tries every candidate on a fresh transport/session
• treats setup rejection or setup-time disconnect as a rejected candidate
• caches a confirmed key by PLC host and port
• supports allow_legacy_key_fallback=False for deterministic no-probing operation
• keeps missing authentication dependencies as an immediate hard failure
• logs key identifiers, never key material

Validation:

• uv run pre-commit run --all-files
• uv run pytest -q --tb=short (2092 passed, 82 skipped)
• uv build --quiet

[gijzelaerr]

This S7CommPlus work is now tracked in gijzelaerr/s7commplus#18. Pull requests cannot be transferred between repositories, so this PR is being closed in favor of the new-repository tracking issue.