T4: Finalized API testing and CI, Fixed checkstyle and minor bugs, Updated documentation

[jag2430]

Finalized automated API testing, CI pipeline, and project documentation.

Changes

API Testing & CI

• Expanded Postman/Newman API tests to cover all endpoints with equivalence partitions
• Fixed variable persistence issue (pm.collectionVariables → pm.environment) for Newman compatibility
• Verified CI pipeline runs all unit, integration, and API tests automatically

Bug Fixes

• Updated deleteTransactionViaGet to return proper HTTP status codes (404/500) instead of 200 with error text
• Fixed zero budget update logic in updateUserJson
• Resolved checkstyle violations

Documentation

• Updated api-testing.md with equivalence partitions documentation
• Expanded README with CI instructions, third-party client guide, and multiple client handling
• Added Javadoc comments where needed

Unit Tests

• Updated RouteControllerTests for new deleteTransactionViaGet behavior
• Expanded test coverage ti ensure coverage for both valid and invalid equivalence partitions for each input