feature: add test infrastructure and developer testing guide (PR 1) (#448)

- 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

---------

Co-authored-by: Copilot <[email protected]>

Author: bakonyiferenc

docs/testing.md

@@ -0,0 +1,161 @@
+# Testing Guide for Developers
+
+This guide covers testing strategies, patterns, and workflows for the Sagemcom API.
+
+## Overview
+
+The test suite uses `pytest` with async support to validate the client's behavior against mocked Sagemcom router API responses. Tests are split into:
+
+- **Unit tests** (`tests/unit/`) - Mock-based tests for individual methods, error handling, and encryption
+- **Integration tests** (`tests/integration/`) - Tests against real or comprehensive simulated router APIs (requires device access)
+- **Fixtures** (`tests/fixtures/`) - Sample API response payloads from different router models
+
+## Running Tests (from Dev Container)
+
+```bash
+# Run all tests
+poetry run pytest
+
+# Run only unit tests
+poetry run pytest tests/unit/
+
+# Run with coverage report
+poetry run pytest --cov=sagemcom_api
+
+# Run with coverage HTML report
+poetry run pytest --cov=sagemcom_api --cov-report=html
+
+# Run specific test file
+poetry run pytest tests/unit/test_client_basic.py
+
+# Run specific test
+poetry run pytest tests/unit/test_client_basic.py::test_login_success
+```
+
+## Test Structure
+
+### Mocking Strategy
+
+We mock at the **`aiohttp.ClientSession.post`** level to:
+- Simulate realistic HTTP interactions
+- Test full request/response cycle including JSON encoding/decoding
+- Validate request payload structure
+- Control response status codes and payloads
+
+### Fixture Patterns
+
+All fixtures are defined in `tests/conftest.py` with **function scope** for test isolation:
+
+- **`mock_session_factory`** - Factory for creating mock aiohttp sessions with custom responses
+- **`login_success_response`** - Mock response for successful login
+- **`login_auth_error_response`** - Mock response for authentication failure
+- **`mock_client_...`** - Pre-configured SagemcomClient with mocked session
+
+Example usage:
+```python
+@pytest.mark.asyncio
+async def test_example(mock_session_factory, login_success_response):
+    mock_session = mock_session_factory([login_success_response])
+    client = SagemcomClient("192.168.1.1", "admin", "password",
+                           EncryptionMethod.MD5, session=mock_session)
+    # Test implementation...
+```
+
+### API Response Fixtures
+
+Realistic API responses are stored in `tests/fixtures/` as JSON files mirroring actual router responses:
+
+- `login_success.json` - Successful login with session_id and nonce
+- `login_auth_error.json` - Authentication failure (XMO_AUTHENTICATION_ERR)
+- `device_info.json` - Device information response
+- `hosts.json` - Connected devices list
+- `xpath_value.json` - Generic XPath query response
+
+These fixtures preserve the actual JSON-RPC structure from Sagemcom routers:
+```json
+{
+  "reply": {
+    "error": {"description": "XMO_REQUEST_NO_ERR"},
+    "actions": [{
+      "callbacks": [{
+        "parameters": {"id": 12345, "nonce": "abcdef123456"}
+      }]
+    }]
+  }
+}
+```
+
+## Testing Patterns
+
+### 1. Testing Authentication
+
+All three encryption methods (MD5, SHA512, MD5_NONCE) must be tested. See `test_client_basic.py` for examples:
+
+```python
+@pytest.mark.asyncio
+async def test_login_success(mock_session_factory, login_success_response):
+    """Test successful login flow."""
+    # Demonstrates mocking login with session_id/nonce exchange
+```
+
+### 2. Testing Error Handling
+
+Each `XMO_*_ERR` constant should have corresponding test cases:
+
+```python
+@pytest.mark.asyncio
+async def test_authentication_error(mock_session_factory, login_auth_error_response):
+    """Test AuthenticationException is raised on XMO_AUTHENTICATION_ERR."""
+    # Demonstrates error response mocking
+```
+
+### 3. Testing XPath Operations
+
+Validate URL encoding with safe characters preserved:
+
+```python
+@pytest.mark.asyncio
+async def test_xpath_url_encoding(mock_session_factory):
+    """Test XPath values are URL-encoded with /=[]' preserved."""
+    # Demonstrates XPath encoding validation
+```
+
+### 4. Testing Sequential Responses
+
+Most API operations require multiple HTTP requests (login → operation). Use `mock_session_factory` with response lists:
+
+```python
+# Two sequential responses
+mock_session = mock_session_factory([login_success_response, xpath_value_response])
+
+await client.login()              # Consumes login_success_response (1st call)
+await client.get_value_by_xpath() # Consumes xpath_value_response (2nd call)
+await client.logout()             # Would raise StopIteration (no 3rd response)
+```
+
+## Adding New Tests
+
+### For a new unit test:
+
+1. Determine what you're testing (method, error case, encryption variant)
+2. Create or reuse fixture for API response in `tests/fixtures/`
+3. Create test file in `tests/unit/test_<module>.py`
+4. Use `mock_session_factory` to inject responses
+5. Write assertions for both success and error paths
+6. Run test with coverage to verify new lines are covered
+
+### For a new integration test:
+
+1. Document router model and firmware version in test docstring
+2. Create test in `tests/integration/test_<feature>.py`
+3. Add conditional skip if router not available: `@pytest.mark.skipif(...)`
+4. Use real credentials from environment variables, not hardcoded
+
+## Test Coverage
+
+Run coverage reports regularly:
+```bash
+poetry run pytest --cov=sagemcom_api --cov-report=term-missing
+```
+
+The `--cov-report=term-missing` shows which lines lack coverage.