Add comprehensive test suite with 97% code coverage

Implemented full test coverage for core bot components:

Test Coverage:
- MentionDetector: 98% (28 tests) - mention detection, nickname handling, content extraction
- Logger: 98% (15 tests) - file logging, log levels, unicode support
- ConversationMemory: 91% (25 tests) - history management, time filtering, JSON persistence
- PerplexityProvider: 100% (22 tests) - API calls, error handling, statistics
- Config: 100% (26 tests) - env loading, validation, JSON config

Infrastructure:
- Added pytest, pytest-asyncio, pytest-mock, pytest-cov to requirements.txt
- Created pytest.ini with coverage configuration
- Created .coveragerc to exclude non-production files
- Added conftest.py with shared fixtures and test isolation

Test Features:
- 116 total tests, all passing
- Isolated test environment (clean env vars, logging handlers)
- Async testing support for PerplexityProvider
- Mocked HTTP requests to avoid real API calls
- Comprehensive edge case coverage
- Unicode/German character support testing

Total: 97% code coverage across 273 statements

tests/conftest.py

@@ -0,0 +1,172 @@
+"""
+Shared pytest fixtures for Eugen Bot tests
+"""
+import pytest
+import tempfile
+import shutil
+import os
+from pathlib import Path
+from datetime import datetime, timedelta
+import json
+
+
+@pytest.fixture(autouse=True)
+def clean_env():
+    """Clean environment variables and logging handlers before each test"""
+    import logging
+
+    # Clean environment variables
+    env_vars_to_clean = [
+        'TWITCH_OAUTH_TOKEN', 'TWITCH_CHANNEL', 'TWITCH_BOT_NICKNAME',
+        'PERPLEXITY_API_KEY', 'PERPLEXITY_MODEL', 'MAX_TOKENS',
+        'DEBUG_MODE', 'AUTO_RECONNECT', 'RECONNECT_DELAY',
+        'CONTEXT_RETENTION_HOURS', 'DATA_DIR', 'LOG_DIR'
+    ]
+
+    # Save original values
+    original_env = {key: os.environ.get(key) for key in env_vars_to_clean}
+
+    # Clear them
+    for key in env_vars_to_clean:
+        os.environ.pop(key, None)
+
+    yield
+
+    # Restore original values
+    for key in env_vars_to_clean:
+        os.environ.pop(key, None)
+        if original_env[key] is not None:
+            os.environ[key] = original_env[key]
+
+    # Clean up logging handlers to prevent test interference
+    for logger_name in ['eugen_main', 'eugen_api']:
+        logger = logging.getLogger(logger_name)
+        logger.handlers.clear()
+        logger.setLevel(logging.NOTSET)
+
+
+@pytest.fixture
+def temp_dir():
+    """Create a temporary directory for tests"""
+    temp_path = Path(tempfile.mkdtemp())
+    yield temp_path
+    # Cleanup
+    if temp_path.exists():
+        shutil.rmtree(temp_path)
+
+
+@pytest.fixture
+def mock_env_file(temp_dir):
+    """Create a mock .env file for testing"""
+    env_path = temp_dir / ".env"
+    env_content = """TWITCH_OAUTH_TOKEN=oauth:test_token_12345
+TWITCH_CHANNEL=#test_channel
+TWITCH_BOT_NICKNAME=TestBot
+PERPLEXITY_API_KEY=pplx-test-key-12345
+PERPLEXITY_MODEL=sonar-pro
+MAX_TOKENS=450
+DEBUG_MODE=true
+AUTO_RECONNECT=true
+RECONNECT_DELAY=10
+CONTEXT_RETENTION_HOURS=1
+"""
+    env_path.write_text(env_content)
+    return env_path
+
+
+@pytest.fixture
+def mock_config_json(temp_dir):
+    """Create a mock config.json file for testing"""
+    config_path = temp_dir / "config.json"
+    config_data = {
+        "bot_name": "TestBot",
+        "model": "sonar-pro",
+        "max_tokens": 450,
+        "debug_mode": True,
+        "auto_reconnect": True,
+        "reconnect_delay": 10,
+        "context_retention_hours": 1
+    }
+    config_path.parent.mkdir(parents=True, exist_ok=True)
+    with open(config_path, 'w') as f:
+        json.dump(config_data, f)
+    return config_path
+
+
+@pytest.fixture
+def sample_conversation_history():
+    """Sample conversation history for testing"""
+    now = datetime.now()
+    return [
+        {
+            "role": "user",
+            "content": "Hello bot!",
+            "timestamp": (now - timedelta(minutes=10)).isoformat()
+        },
+        {
+            "role": "assistant",
+            "content": "Hi! How can I help?",
+            "timestamp": (now - timedelta(minutes=9)).isoformat()
+        },
+        {
+            "role": "user",
+            "content": "What's the weather?",
+            "timestamp": (now - timedelta(minutes=5)).isoformat()
+        },
+        {
+            "role": "assistant",
+            "content": "I don't have weather info.",
+            "timestamp": (now - timedelta(minutes=4)).isoformat()
+        }
+    ]
+
+
+@pytest.fixture
+def old_conversation_history():
+    """Old conversation history (beyond retention) for testing"""
+    old_time = datetime.now() - timedelta(hours=2)
+    return [
+        {
+            "role": "user",
+            "content": "Old message",
+            "timestamp": old_time.isoformat()
+        },
+        {
+            "role": "assistant",
+            "content": "Old response",
+            "timestamp": (old_time + timedelta(minutes=1)).isoformat()
+        }
+    ]
+
+
+@pytest.fixture
+def mock_perplexity_response():
+    """Mock successful Perplexity API response"""
+    return {
+        "id": "test-completion-123",
+        "model": "sonar-pro",
+        "choices": [
+            {
+                "index": 0,
+                "message": {
+                    "role": "assistant",
+                    "content": "This is a test response from the AI."
+                },
+                "finish_reason": "stop"
+            }
+        ],
+        "usage": {
+            "prompt_tokens": 50,
+            "completion_tokens": 20,
+            "total_tokens": 70
+        }
+    }
+
+
+@pytest.fixture
+def sample_messages():
+    """Sample messages for API testing"""
+    return [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"role": "user", "content": "Hello, how are you?"}
+    ]