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_banks.py

@@ -0,0 +1,154 @@
+"""Tests for banks API endpoints."""
+import pytest
+import respx
+import httpx
+from unittest.mock import patch
+
+from leggend.services.gocardless_service import GoCardlessService
+
+
+@pytest.mark.api
+class TestBanksAPI:
+    """Test bank-related API endpoints."""
+    
+    @respx.mock
+    def test_get_institutions_success(self, api_client, mock_config, mock_auth_token, sample_bank_data):
+        """Test successful retrieval of bank institutions."""
+        # Mock GoCardless token creation/refresh
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/token/new/").mock(
+            return_value=httpx.Response(200, json={"access": "test-token", "refresh": "test-refresh"})
+        )
+        
+        # Mock GoCardless institutions API
+        respx.get("https://bankaccountdata.gocardless.com/api/v2/institutions/").mock(
+            return_value=httpx.Response(200, json=sample_bank_data)
+        )
+        
+        with patch('leggend.config.config', mock_config):
+            response = api_client.get("/api/v1/banks/institutions?country=PT")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["success"] is True
+        assert len(data["data"]) == 2
+        assert data["data"][0]["id"] == "REVOLUT_REVOLT21"
+        assert data["data"][1]["id"] == "BANCOBPI_BBPIPTPL"
+
+    @respx.mock
+    def test_get_institutions_invalid_country(self, api_client, mock_config):
+        """Test institutions endpoint with invalid country code."""
+        # Mock GoCardless token creation
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/token/new/").mock(
+            return_value=httpx.Response(200, json={"access": "test-token", "refresh": "test-refresh"})
+        )
+        
+        # Mock empty institutions response for invalid country
+        respx.get("https://bankaccountdata.gocardless.com/api/v2/institutions/").mock(
+            return_value=httpx.Response(200, json=[])
+        )
+        
+        with patch('leggend.config.config', mock_config):
+            response = api_client.get("/api/v1/banks/institutions?country=XX")
+        
+        # Should still work but return empty or filtered results
+        assert response.status_code in [200, 404]
+
+    @respx.mock
+    def test_connect_to_bank_success(self, api_client, mock_config, mock_auth_token):
+        """Test successful bank connection creation."""
+        requisition_data = {
+            "id": "req-123",
+            "institution_id": "REVOLUT_REVOLT21",
+            "status": "CR",
+            "created": "2025-09-02T00:00:00Z",
+            "link": "https://example.com/auth"
+        }
+        
+        # Mock GoCardless token creation
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/token/new/").mock(
+            return_value=httpx.Response(200, json={"access": "test-token", "refresh": "test-refresh"})
+        )
+        
+        # Mock GoCardless requisitions API
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/requisitions/").mock(
+            return_value=httpx.Response(200, json=requisition_data)
+        )
+        
+        request_data = {
+            "institution_id": "REVOLUT_REVOLT21",
+            "redirect_url": "http://localhost:8000/"
+        }
+        
+        with patch('leggend.config.config', mock_config):
+            response = api_client.post("/api/v1/banks/connect", json=request_data)
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["success"] is True
+        assert data["data"]["id"] == "req-123"
+        assert data["data"]["institution_id"] == "REVOLUT_REVOLT21"
+
+    @respx.mock
+    def test_get_bank_status_success(self, api_client, mock_config, mock_auth_token):
+        """Test successful retrieval of bank connection status."""
+        requisitions_data = {
+            "results": [
+                {
+                    "id": "req-123",
+                    "institution_id": "REVOLUT_REVOLT21", 
+                    "status": "LN",
+                    "created": "2025-09-02T00:00:00Z",
+                    "accounts": ["acc-123"]
+                }
+            ]
+        }
+        
+        # Mock GoCardless token creation
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/token/new/").mock(
+            return_value=httpx.Response(200, json={"access": "test-token", "refresh": "test-refresh"})
+        )
+        
+        # Mock GoCardless requisitions API
+        respx.get("https://bankaccountdata.gocardless.com/api/v2/requisitions/").mock(
+            return_value=httpx.Response(200, json=requisitions_data)
+        )
+        
+        with patch('leggend.config.config', mock_config):
+            response = api_client.get("/api/v1/banks/status")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["success"] is True
+        assert len(data["data"]) == 1
+        assert data["data"][0]["bank_id"] == "REVOLUT_REVOLT21"
+        assert data["data"][0]["status_display"] == "LINKED"
+
+    def test_get_supported_countries(self, api_client):
+        """Test supported countries endpoint."""
+        response = api_client.get("/api/v1/banks/countries")
+        
+        assert response.status_code == 200
+        data = response.json()
+        assert data["success"] is True
+        assert len(data["data"]) > 0
+        
+        # Check some expected countries
+        country_codes = [country["code"] for country in data["data"]]
+        assert "PT" in country_codes
+        assert "GB" in country_codes
+        assert "DE" in country_codes
+
+    @respx.mock
+    def test_authentication_failure(self, api_client, mock_config):
+        """Test handling of authentication failures."""
+        # Mock token creation failure
+        respx.post("https://bankaccountdata.gocardless.com/api/v2/token/new/").mock(
+            return_value=httpx.Response(401, json={"detail": "Invalid credentials"})
+        )
+        
+        with patch('leggend.config.config', mock_config):
+            response = api_client.get("/api/v1/banks/institutions")
+        
+        assert response.status_code == 500
+        data = response.json()
+        assert "Failed to get institutions" in data["detail"]