Add @DisplayName and @Nested class grouping to all tests

[rammrain]

Summary

Improve test readability by adding JUnit 5 @DisplayName annotations to every test class and method, and grouping related tests using @Nested inner classes.

Requirements

• Add @DisplayName to every test class and test method with a clear, human-readable description
• Group related test methods into @Nested inner classes (e.g., by method under test, scenario category, or happy/sad path)
• Apply consistently across all existing and new tests
• Follow this pattern going forward for all issues that include test work

Example

@DisplayName("OrderService")
class OrderServiceTest {

    @Nested
    @DisplayName("createOrder")
    class CreateOrder {

        @Test
        @DisplayName("returns payment URL on success")
        void returnsPaymentUrlOnSuccess() { ... }

        @Test
        @DisplayName("throws validation exception when amount is missing")
        void throwsWhenAmountMissing() { ... }
    }
}

Testing

This issue is about tests — apply to all test classes created by other issues. Retrofit the existing MontonioSdkConfigurationTest as well.