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_api_client.py

@@ -0,0 +1,150 @@
+"""Tests for CLI API client."""
+import pytest
+import requests_mock
+from unittest.mock import patch
+
+from leggen.api_client import LeggendAPIClient
+
+
+@pytest.mark.cli
+class TestLeggendAPIClient:
+    """Test the CLI API client."""
+
+    def test_health_check_success(self):
+        """Test successful health check."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/health", json={"status": "healthy"})
+            
+            result = client.health_check()
+            assert result is True
+
+    def test_health_check_failure(self):
+        """Test health check failure."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/health", status_code=500)
+            
+            result = client.health_check()
+            assert result is False
+
+    def test_get_institutions_success(self, sample_bank_data):
+        """Test getting institutions via API client."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        api_response = {
+            "success": True,
+            "data": sample_bank_data,
+            "message": "Found 2 institutions for PT"
+        }
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/api/v1/banks/institutions", json=api_response)
+            
+            result = client.get_institutions("PT")
+            assert len(result) == 2
+            assert result[0]["id"] == "REVOLUT_REVOLT21"
+
+    def test_get_accounts_success(self, sample_account_data):
+        """Test getting accounts via API client."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        api_response = {
+            "success": True,
+            "data": [sample_account_data],
+            "message": "Retrieved 1 accounts"
+        }
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/api/v1/accounts", json=api_response)
+            
+            result = client.get_accounts()
+            assert len(result) == 1
+            assert result[0]["id"] == "test-account-123"
+
+    def test_trigger_sync_success(self):
+        """Test triggering sync via API client."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        api_response = {
+            "success": True,
+            "data": {"sync_started": True, "force": False},
+            "message": "Started sync for all accounts"
+        }
+        
+        with requests_mock.Mocker() as m:
+            m.post("http://localhost:8000/api/v1/sync", json=api_response)
+            
+            result = client.trigger_sync()
+            assert result["sync_started"] is True
+
+    def test_connection_error_handling(self):
+        """Test handling of connection errors."""
+        client = LeggendAPIClient("http://localhost:9999")  # Non-existent service
+        
+        with pytest.raises(Exception):
+            client.get_accounts()
+
+    def test_http_error_handling(self):
+        """Test handling of HTTP errors."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/api/v1/accounts", status_code=500, 
+                  json={"detail": "Internal server error"})
+            
+            with pytest.raises(Exception):
+                client.get_accounts()
+
+    def test_custom_api_url(self):
+        """Test using custom API URL."""
+        custom_url = "http://custom-host:9000"
+        client = LeggendAPIClient(custom_url)
+        
+        assert client.base_url == custom_url
+
+    def test_environment_variable_url(self):
+        """Test using environment variable for API URL."""
+        with patch.dict('os.environ', {'LEGGEND_API_URL': 'http://env-host:7000'}):
+            client = LeggendAPIClient()
+            assert client.base_url == "http://env-host:7000"
+
+    def test_sync_with_options(self):
+        """Test sync with various options."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        api_response = {
+            "success": True,
+            "data": {"sync_started": True, "force": True},
+            "message": "Started sync for 2 specific accounts"
+        }
+        
+        with requests_mock.Mocker() as m:
+            m.post("http://localhost:8000/api/v1/sync", json=api_response)
+            
+            result = client.trigger_sync(account_ids=["acc1", "acc2"], force=True)
+            assert result["sync_started"] is True
+            assert result["force"] is True
+
+    def test_get_scheduler_config(self):
+        """Test getting scheduler configuration."""
+        client = LeggendAPIClient("http://localhost:8000")
+        
+        api_response = {
+            "success": True,
+            "data": {
+                "enabled": True,
+                "hour": 3,
+                "minute": 0,
+                "next_scheduled_sync": "2025-09-03T03:00:00Z"
+            }
+        }
+        
+        with requests_mock.Mocker() as m:
+            m.get("http://localhost:8000/api/v1/sync/scheduler", json=api_response)
+            
+            result = client.get_scheduler_config()
+            assert result["enabled"] is True
+            assert result["hour"] == 3