Add end-to-end test suite for real PLC testing

Adds test_client_e2e.py with comprehensive tests against a real Siemens
S7 PLC. Tests are marked with @pytest.mark.e2e and require:
- A real PLC connection (configure IP, rack, slot at top of file)
- Two data blocks: DB1 (read-only) and DB2 (read-write)

Run with: pytest tests/test_client_e2e.py -m e2e

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gijzelaerr

pyproject.toml

@@ -50,6 +50,7 @@ testpaths = ["tests"]
 markers =[
     "client",
     "common",
+    "e2e: end-to-end tests requiring a real PLC connection",
     "logo",
     "mainloop",
     "partner",