Implement Strategy Pattern, Cache Preload, Unit & Integration Tests

[AchmadSu]

Summary

This PR implements the required architecture for the finance application:

• Strategy Pattern for IDRDataFetcher
• Cache layer to avoid repeated external API calls
• ApplicationRunner to preload external API data at startup
• ExternalApiService to encapsulate WebClient logic
• Unit tests for all strategy implementations
• Integration test for ApplicationRunner preload process

Key Improvements

• Clean separation of concerns (Runner → Service → WebClient)
• External API calls are mocked in tests
• No external dependency during test execution
• Spread calculation logic covered by unit test

Test Coverage

• Unit tests:
  • LatestRateStrategy
  • HistoricalRateStrategy
  • SupportedCurrenciesStrategy
• Integration test:
  • DataPreloadRunner

How To Run

• mvn clean install
• Run application with mvn spring-boot:run
• Run testing with mvn test
• Access endpoint: http://localhost:8080/api/finance/data/{type}

Notes

WebClient configuration added via WebClientConfig.