fix(s7commplus): encode generated SessionKey descriptor

[gijzelaerr]

Summary

• pass the newly generated SessionKey directly into SecurityKey descriptor encoding
• derive the symmetric KeyId from that pending key rather than the inactive connection field
• reject missing public or symmetric key material instead of emitting plausible zero-key descriptors
• add deterministic descriptor regression coverage

Validation

• uv run --frozen --extra test --extra s7commplus pytest -q (1,910 passed, 78 skipped)
• uv run --frozen pre-commit run --all-files
• uv build --no-sources

Fixes #830