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>

pyproject.toml

@@ -49,6 +49,11 @@ leggend = "leggend.main:main"
 dev = [
     "ruff>=0.6.1",
     "pre-commit>=3.6.0",
+    "pytest>=8.0.0",
+    "pytest-asyncio>=0.23.0",
+    "pytest-mock>=3.12.0", 
+    "respx>=0.21.0",
+    "requests-mock>=1.12.0",
 ]
 
 [tool.hatch.build.targets.sdist]
@@ -64,3 +69,24 @@ build-backend = "hatchling.build"
 [tool.ruff]
 lint.ignore = ["E501", "B008", "B006"]
 lint.extend-select = ["B", "C4", "PIE", "T20", "SIM", "TCH"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = "test_*.py"
+python_classes = "Test*" 
+python_functions = "test_*"
+addopts = [
+    "-v",
+    "--tb=short", 
+    "--strict-markers",
+    "--disable-warnings"
+]
+asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "function"
+markers = [
+    "unit: Unit tests",
+    "integration: Integration tests",
+    "slow: Slow running tests", 
+    "api: API endpoint tests",
+    "cli: CLI command tests"
+]