Add unit tests for low_level S7Client, S7Server, and S7Partner modules

[Copilot]

This PR adds comprehensive unit tests for the pure Python implementations in the snap7/low_level/ directory, specifically for s7_client.py, s7_server.py, and s7_partner.py.

Overview

The low_level directory contains native Python implementations of the S7 protocol (similar to Sharp7), which provide an alternative to the C library wrapper. These implementations previously lacked dedicated unit tests.

Changes

New Test Files

1. tests/test_low_level_s7_client.py - 11 tests covering:

   • S7Client initialization and configuration
   • Connection parameter setup (set_connection_params)
   • Parameter management (get_param, set_param)
   • Helper classes (S7SZL, S7SZLHeader)
   • Basic operations (disconnect, get_last_error, get_exec_time)

2. tests/test_low_level_s7_server.py - 12 tests covering:

   • S7Server initialization and state management
   • Memory area registration/unregistration
   • Server status retrieval and CPU state management
   • Start/stop operations on custom ports
   • Parameter configuration
   • Event callback setup

3. tests/test_low_level_s7_partner.py - 20 tests covering:

   • S7Partner initialization (both active and passive modes)
   • Lifecycle management (create, start, stop, destroy)
   • Asynchronous operation completion checking
   • Parameter management and statistics retrieval
   • Send/receive buffer operations
   • Callback configuration

Configuration Updates

• Updated pyproject.toml to add pytest markers for the new test modules: low_level_client, low_level_server, low_level_partner

Test Coverage

All 43 new tests pass successfully and cover:

• Core functionality of the pure Python S7 implementations
• Initialization and configuration
• State management and lifecycle operations
• Parameter handling
• Buffer and callback management
• Error handling

The tests are designed to run without requiring an actual PLC connection or the native snap7 library, making them suitable for CI/CD pipelines and local development.

Testing

# Run all low_level tests
pytest tests/test_low_level_*.py -v

# Run specific module tests
pytest tests/test_low_level_s7_client.py -v
pytest tests/test_low_level_s7_server.py -v
pytest tests/test_low_level_s7_partner.py -v

Fixes #[issue_number]

Original prompt

Ok, add the unit test in folder test for low_level/ S7_client and s7_server and s7_partner.

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