fix: Make description field optional in ad_defaults

The description field in the main configuration (ad_defaults) is now optional.
Previously, the bot would fail if no description or affixes were provided in
the main configuration. This change addresses issue #435.

Changes:
- Add fallback to empty string ("") when all description prefix/suffix sources
  are None in __get_description_with_affixes method
- Add comprehensive test suite for description handling in test_init.py
- Fix coverage path in pyproject.toml from 'kleinanzeigen_bot' to
  'src/kleinanzeigen_bot'

New tests cover:
- Description handling without main config description
- New format affixes in configuration
- Mixed old/new format affixes
- Ad-level affix precedence
- None value handling in affixes
- Email address @ symbol replacement

This change maintains backward compatibility while making the description
field optional in the main configuration, improving flexibility for users.

pyproject.toml

@@ -253,9 +253,9 @@ addopts = """
   --ignore=data
   --ignore=dist
   --ignore=src/kleinanzeigen_bot/__main__.py
-  --cov=kleinanzeigen_bot
+  --cov=src/kleinanzeigen_bot
   --cov-report=term-missing
-  """
+"""
 markers = [
   "itest: marks a test as an integration test (i.e. a test with external dependencies)",
   "asyncio: mark test as async"