chore: update development dependencies and improve test coverage

- Upgraded `pytest-asyncio` to version 1.0 to ensure compatibility with the latest features and improvements.
- Increased the coverage threshold in pytest configuration to 80%, enhancing the quality assurance process.
- Added a new `conftest.py` file to manage shared fixtures and improve test organization.
- Introduced multiple new test files to cover various components, ensuring comprehensive test coverage across the application.
- Updated the `.coverage` file to reflect the latest coverage metrics.

tests/test_handlers_init.py

@@ -0,0 +1,13 @@
+"""Tests for duty_teller.handlers.register_handlers."""
+
+from unittest.mock import MagicMock
+
+from duty_teller.handlers import register_handlers
+
+
+def test_register_handlers_adds_all_handlers():
+    """register_handlers: adds command, import, group pin handlers and error handler."""
+    mock_app = MagicMock()
+    register_handlers(mock_app)
+    assert mock_app.add_handler.call_count >= 9
+    assert mock_app.add_error_handler.call_count == 1