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

requirements.txt

@@ -3,3 +3,9 @@ python-dotenv==1.0.0
 FreeSimpleGUI==5.1.1
 requests==2.31.0
 httpx==0.25.0
+
+# Testing dependencies
+pytest==7.4.3
+pytest-asyncio==0.21.1
+pytest-mock==3.12.0
+pytest-cov==4.1.0