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/conftest.py

@@ -0,0 +1,138 @@
+"""Pytest configuration and shared fixtures."""
+import pytest
+import tempfile
+import json
+from pathlib import Path
+from unittest.mock import patch
+from fastapi.testclient import TestClient
+
+from leggend.main import create_app
+from leggend.config import Config
+
+
+@pytest.fixture
+def temp_config_dir():
+    """Create a temporary config directory for testing."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        config_dir = Path(tmpdir) / ".config" / "leggen"
+        config_dir.mkdir(parents=True, exist_ok=True)
+        yield config_dir
+
+
+@pytest.fixture 
+def mock_config(temp_config_dir):
+    """Mock configuration for testing."""
+    config_data = {
+        "gocardless": {
+            "key": "test-key",
+            "secret": "test-secret", 
+            "url": "https://bankaccountdata.gocardless.com/api/v2"
+        },
+        "database": {
+            "sqlite": True,
+            "mongodb": False
+        },
+        "scheduler": {
+            "sync": {
+                "enabled": True,
+                "hour": 3,
+                "minute": 0
+            }
+        }
+    }
+    
+    config_file = temp_config_dir / "config.toml"
+    with open(config_file, "wb") as f:
+        import tomli_w
+        tomli_w.dump(config_data, f)
+    
+    # Mock the config path
+    with patch.object(Config, 'load_config') as mock_load:
+        mock_load.return_value = config_data
+        config = Config()
+        config._config = config_data
+        config._config_path = str(config_file)
+        yield config
+
+
+@pytest.fixture
+def mock_auth_token(temp_config_dir):
+    """Mock GoCardless authentication token."""
+    auth_data = {
+        "access": "mock-access-token",
+        "refresh": "mock-refresh-token"
+    }
+    
+    auth_file = temp_config_dir / "auth.json" 
+    with open(auth_file, "w") as f:
+        json.dump(auth_data, f)
+    
+    return auth_data
+
+
+@pytest.fixture
+def fastapi_app():
+    """Create FastAPI test application."""
+    return create_app()
+
+
+@pytest.fixture
+def api_client(fastapi_app):
+    """Create FastAPI test client."""
+    return TestClient(fastapi_app)
+
+
+@pytest.fixture
+def sample_bank_data():
+    """Sample bank/institution data for testing."""
+    return [
+        {
+            "id": "REVOLUT_REVOLT21",
+            "name": "Revolut",
+            "bic": "REVOLT21", 
+            "transaction_total_days": 90,
+            "countries": ["GB", "LT"]
+        },
+        {
+            "id": "BANCOBPI_BBPIPTPL",
+            "name": "Banco BPI",
+            "bic": "BBPIPTPL",
+            "transaction_total_days": 90, 
+            "countries": ["PT"]
+        }
+    ]
+
+
+@pytest.fixture
+def sample_account_data():
+    """Sample account data for testing."""
+    return {
+        "id": "test-account-123",
+        "institution_id": "REVOLUT_REVOLT21", 
+        "status": "READY",
+        "iban": "LT313250081177977789",
+        "created": "2024-02-13T23:56:00Z",
+        "last_accessed": "2025-09-01T09:30:00Z"
+    }
+
+
+@pytest.fixture
+def sample_transaction_data():
+    """Sample transaction data for testing.""" 
+    return {
+        "transactions": {
+            "booked": [
+                {
+                    "internalTransactionId": "txn-123",
+                    "bookingDate": "2025-09-01",
+                    "valueDate": "2025-09-01", 
+                    "transactionAmount": {
+                        "amount": "-10.50",
+                        "currency": "EUR"
+                    },
+                    "remittanceInformationUnstructured": "Coffee Shop Payment"
+                }
+            ],
+            "pending": []
+        }
+    }