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_errors.py

@@ -0,0 +1,53 @@
+"""Tests for duty_teller.handlers.errors (error_handler)."""
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from duty_teller.handlers.errors import error_handler
+
+
+@pytest.mark.asyncio
+async def test_error_handler_replies_with_generic_message():
+    """error_handler: when update has effective_message, reply_text with errors.generic."""
+    # Handler checks isinstance(update, Update); patch Update so our mock passes.
+    class FakeUpdate:
+        pass
+
+    update = FakeUpdate()
+    update.effective_message = MagicMock()
+    update.effective_message.reply_text = AsyncMock()
+    update.effective_user = MagicMock()
+    context = MagicMock()
+    context.error = Exception("test error")
+
+    with patch("duty_teller.handlers.errors.Update", FakeUpdate):
+        with patch("duty_teller.handlers.errors.get_lang", return_value="en"):
+            with patch("duty_teller.handlers.errors.t") as mock_t:
+                mock_t.return_value = "An error occurred."
+                await error_handler(update, context)
+    update.effective_message.reply_text.assert_called_once_with("An error occurred.")
+    mock_t.assert_called_with("en", "errors.generic")
+
+
+@pytest.mark.asyncio
+async def test_error_handler_no_effective_message_does_not_send():
+    """error_handler: when update has no effective_message, does not call reply_text."""
+    update = MagicMock()
+    update.effective_message = None
+    update.effective_user = MagicMock()
+    context = MagicMock()
+    context.error = Exception("test")
+
+    with patch("duty_teller.handlers.errors.get_lang", return_value="en"):
+        with patch("duty_teller.handlers.errors.t") as mock_t:
+            await error_handler(update, context)
+    mock_t.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_error_handler_update_none_does_not_crash():
+    """error_handler: when update is None, does not crash (no reply)."""
+    context = MagicMock()
+    context.error = Exception("test")
+    await error_handler(None, context)