feature: add test infrastructure and developer testing guide (PR 1)

[bakonyiferenc]

• Add developer testing guide in docs/testing.md
• Implement pytest fixtures in tests/conftest.py:
  • mock_session_factory for sequential response mocking
  • Response fixtures for login, errors, device info, hosts, xpath
  • Pre-configured client fixtures for all encryption methods
• Add API response fixtures (6 JSON files) with realistic router responses
• Add 4 example tests demonstrating key patterns:
  • Successful login with session state validation
  • Authentication error handling
  • XPath operations with sequential responses
  • Pre-configured fixture usage
• Configure pytest in pyproject.toml:
  • Add pytest-asyncio, pytest-aiohttp, pytest-cov dependencies
  • Set asyncio_mode, testpaths, and markers

Part of #447: Add unit and integration tests to have test coverage

[Copilot]

Pull request overview

This PR establishes comprehensive test infrastructure for the Sagemcom API client, including pytest configuration, mock fixtures, example tests, and developer documentation. The changes enable unit and integration testing with realistic API response mocking.

Key Changes:

• Added pytest configuration with asyncio support and coverage tooling
• Implemented mock session factory and response fixtures for authentication, errors, and data operations
• Created 6 JSON fixture files containing realistic router API responses
• Provided 4 example tests demonstrating key testing patterns

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pyproject.toml	Added pytest dependencies (pytest-asyncio, pytest-aiohttp, pytest-cov) and configuration for async tests, test paths, and custom markers
poetry.lock	Updated dependency lock file with new test-related packages and version constraints
tests/conftest.py	Implemented shared fixtures including mock session factory, response fixtures for various API scenarios, and pre-configured client fixtures for different encryption methods
tests/fixtures/login_success.json	Realistic API response for successful authentication with session ID and nonce
tests/fixtures/login_auth_error.json	API response demonstrating authentication failure error handling
tests/fixtures/login_invalid_session.json	API response for invalid session error scenario
tests/fixtures/device_info.json	Comprehensive device information response with 993 lines covering hardware, software, temperature sensors, and system configuration
tests/fixtures/hosts.json	Connected hosts/devices list response with detailed device metadata
tests/fixtures/xpath_value.json	Generic XPath query response demonstrating WiFi radio status retrieval
tests/unit/test_client_basic.py	Four example tests demonstrating mocking patterns for login success/failure, XPath operations, and pre-configured fixtures
docs/testing.md	Developer guide covering test structure, mocking strategies, fixture patterns, and instructions for running and adding tests

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[iMicknl]

Looks good to me! Thank you!

(later we will need to add this to GitHub Actions as well)