Protocol conformance test suite

[gijzelaerr]

Summary

Create a dedicated test suite that validates the library's protocol implementation against the S7 protocol specification, covering packet structure, error codes, and edge cases.

Background

The current tests validate functionality (read/write works, server responds correctly) but don't systematically verify protocol-level correctness. A conformance suite would test that every PDU is correctly formed according to the protocol specification.

What needs to be done

• Collect reference packet captures from known-good implementations (Wireshark captures from real PLCs)
• Create test cases that verify PDU encoding matches expected byte sequences
• Create test cases that verify PDU decoding handles all valid packet variations
• Test all S7 error codes are correctly parsed and mapped
• Test TPKT framing edge cases (fragmentation, maximum sizes)
• Test COTP connection establishment variations
• Test PDU size negotiation boundary conditions
• Test malformed packet handling (fuzzing-style tests for robustness)
• Verify multi-packet (fragmented) transfer assembly
• Add as a separate pytest marker (e.g., pytest -m conformance)

Notes

• Wireshark's S7 dissector is a good reference for expected packet formats
• Could use captured pcap files as test fixtures
• This would also help validate S7CommPlus protocol implementation as it develops

[gijzelaerr]

Implementation submitted in #633 — adds 70 protocol conformance tests validating TPKT, COTP, and S7 protocol encoding at byte level.