feat: Add comprehensive test suite with 46 passing tests

- Add pytest configuration in pyproject.toml with markers and async support
- Create shared test fixtures in tests/conftest.py for config, auth, and sample data
- Implement unit tests for all major components:
  * Configuration management (11 tests) - TOML loading/saving, singleton pattern
  * FastAPI API endpoints (12 tests) - Banks, accounts, transactions with mocks
  * CLI API client (11 tests) - HTTP client integration and error handling
  * Background scheduler (12 tests) - APScheduler job management and async ops

- Fix GoCardless API authentication mocking by adding token endpoints
- Resolve TOML file writing issues (binary vs text mode for tomli_w)
- Add comprehensive testing documentation to README
- Update code structure documentation to include test organization

Testing framework includes:
- respx for HTTP request mocking
- pytest-asyncio for async test support
- pytest-mock for advanced mocking capabilities
- requests-mock for CLI HTTP client testing
- Realistic test data fixtures for banks, accounts, and transactions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/unit/test_config.py

@@ -0,0 +1,202 @@
+"""Tests for configuration management."""
+import pytest
+import tempfile
+from pathlib import Path
+from unittest.mock import patch
+
+from leggend.config import Config
+
+
+@pytest.mark.unit
+class TestConfig:
+    """Test configuration management."""
+
+    def test_singleton_behavior(self):
+        """Test that Config is a singleton."""
+        config1 = Config()
+        config2 = Config()
+        assert config1 is config2
+
+    def test_load_config_success(self, temp_config_dir):
+        """Test successful configuration loading."""
+        config_data = {
+            "gocardless": {
+                "key": "test-key",
+                "secret": "test-secret",
+                "url": "https://test.example.com"
+            },
+            "database": {
+                "sqlite": True
+            }
+        }
+        
+        config_file = temp_config_dir / "config.toml"
+        with open(config_file, "wb") as f:
+            import tomli_w
+            tomli_w.dump(config_data, f)
+        
+        config = Config()
+        # Reset singleton state for testing
+        config._config = None
+        config._config_path = None
+        
+        result = config.load_config(str(config_file))
+        
+        assert result == config_data
+        assert config.gocardless_config["key"] == "test-key"
+        assert config.database_config["sqlite"] is True
+
+    def test_load_config_file_not_found(self):
+        """Test handling of missing configuration file."""
+        config = Config()
+        config._config = None  # Reset for test
+        
+        with pytest.raises(FileNotFoundError):
+            config.load_config("/nonexistent/config.toml")
+
+    def test_save_config_success(self, temp_config_dir):
+        """Test successful configuration saving."""
+        config_data = {
+            "gocardless": {
+                "key": "new-key",
+                "secret": "new-secret"
+            }
+        }
+        
+        config_file = temp_config_dir / "new_config.toml"
+        config = Config()
+        config._config = None
+        
+        config.save_config(config_data, str(config_file))
+        
+        # Verify file was created and contains correct data
+        assert config_file.exists()
+        
+        import tomllib
+        with open(config_file, "rb") as f:
+            saved_data = tomllib.load(f)
+        
+        assert saved_data == config_data
+
+    def test_update_config_success(self, temp_config_dir):
+        """Test updating configuration values."""
+        initial_config = {
+            "gocardless": {"key": "old-key"},
+            "database": {"sqlite": True}
+        }
+        
+        config_file = temp_config_dir / "config.toml"
+        with open(config_file, "wb") as f:
+            import tomli_w
+            tomli_w.dump(initial_config, f)
+        
+        config = Config()
+        config._config = None
+        config.load_config(str(config_file))
+        
+        config.update_config("gocardless", "key", "new-key")
+        
+        assert config.gocardless_config["key"] == "new-key"
+        
+        # Verify it was saved to file
+        import tomllib
+        with open(config_file, "rb") as f:
+            saved_data = tomllib.load(f)
+        assert saved_data["gocardless"]["key"] == "new-key"
+
+    def test_update_section_success(self, temp_config_dir):
+        """Test updating entire configuration section."""
+        initial_config = {
+            "database": {"sqlite": True, "mongodb": False}
+        }
+        
+        config_file = temp_config_dir / "config.toml"
+        with open(config_file, "wb") as f:
+            import tomli_w
+            tomli_w.dump(initial_config, f)
+        
+        config = Config()
+        config._config = None
+        config.load_config(str(config_file))
+        
+        new_db_config = {"sqlite": False, "mongodb": True, "uri": "mongodb://localhost"}
+        config.update_section("database", new_db_config)
+        
+        assert config.database_config == new_db_config
+
+    def test_scheduler_config_defaults(self):
+        """Test scheduler configuration with defaults."""
+        config = Config()
+        config._config = {}  # Empty config
+        
+        scheduler_config = config.scheduler_config
+        
+        assert scheduler_config["sync"]["enabled"] is True
+        assert scheduler_config["sync"]["hour"] == 3
+        assert scheduler_config["sync"]["minute"] == 0
+        assert scheduler_config["sync"]["cron"] is None
+
+    def test_scheduler_config_custom(self):
+        """Test scheduler configuration with custom values."""
+        custom_config = {
+            "scheduler": {
+                "sync": {
+                    "enabled": False,
+                    "hour": 6,
+                    "minute": 30,
+                    "cron": "0 6 * * 1-5"
+                }
+            }
+        }
+        
+        config = Config()
+        config._config = custom_config
+        
+        scheduler_config = config.scheduler_config
+        
+        assert scheduler_config["sync"]["enabled"] is False
+        assert scheduler_config["sync"]["hour"] == 6
+        assert scheduler_config["sync"]["minute"] == 30
+        assert scheduler_config["sync"]["cron"] == "0 6 * * 1-5"
+
+    def test_environment_variable_config_path(self):
+        """Test using environment variable for config path."""
+        with patch.dict('os.environ', {'LEGGEN_CONFIG_FILE': '/custom/path/config.toml'}):
+            config = Config()
+            config._config = None
+            
+            with patch('builtins.open', side_effect=FileNotFoundError):
+                with pytest.raises(FileNotFoundError):
+                    config.load_config()
+
+    def test_notifications_config(self):
+        """Test notifications configuration access.""" 
+        custom_config = {
+            "notifications": {
+                "discord": {"webhook": "https://discord.webhook", "enabled": True},
+                "telegram": {"token": "bot-token", "chat_id": 123}
+            }
+        }
+        
+        config = Config()
+        config._config = custom_config
+        
+        notifications = config.notifications_config
+        assert notifications["discord"]["webhook"] == "https://discord.webhook"
+        assert notifications["telegram"]["token"] == "bot-token"
+
+    def test_filters_config(self):
+        """Test filters configuration access."""
+        custom_config = {
+            "filters": {
+                "case-insensitive": {"salary": "SALARY", "bills": "BILL"},
+                "amount_threshold": 100.0
+            }
+        }
+        
+        config = Config()
+        config._config = custom_config
+        
+        filters = config.filters_config
+        assert filters["case-insensitive"]["salary"] == "SALARY"
+        assert filters["amount_threshold"] == 100.0