bug: Modified tests for bean validation. (WIP) (Close #6)

Implemented bean validation for `TimeEntryDTO` & `TimeBillingDTO`.

> Note: The tests are passing because we're providing null values for `TimeEntryDTO#hourlyRate`, whereas we should write tests for both scenarios.

src/main/java/com/seyed/ali/timeentryservice/model/dto/TimeBillingDTO.java

@@ -1,6 +1,5 @@
 package com.seyed.ali.timeentryservice.model.dto;
 
-import com.seyed.ali.timeentryservice.annotation.OptionalField;
 import com.seyed.ali.timeentryservice.model.domain.TimeEntry;
 import io.swagger.v3.oas.annotations.media.Schema;
 import jakarta.validation.constraints.DecimalMax;
@@ -15,13 +14,11 @@
 @Builder
 public record TimeBillingDTO(
         @Schema(requiredMode = Schema.RequiredMode.NOT_REQUIRED, description = "A flag determining this time entry is billable", example = "true")
-        @OptionalField
         boolean billable,
 
         @Schema(requiredMode = Schema.RequiredMode.NOT_REQUIRED, description = "The hourly rate in BigDecimal format", example = "10.0")
         @DecimalMin(value = "0.0", inclusive = false, message = "hourlyRate must be greater than 0")
         @DecimalMax(value = "999.99", message = "hourlyRate must be less than 1000")
-        @OptionalField
         BigDecimal hourlyRate
 ) {
 }

src/test/java/com/seyed/ali/timeentryservice/controller/TimeEntryControllerTest.java

@@ -161,7 +161,7 @@ public void getUsersTimeEntryTest() throws Exception {
     @Test
     public void addTimeEntryManuallyTest() throws Exception {
         // given
-        TimeEntryDTO timeEntryDTO = this.timeEntries.getFirst();
+        TimeEntryDTO timeEntryDTO = new TimeEntryDTO("1", "2024-05-11 08:00:00", "2024-05-11 10:00:00", false, null, "02:00:00");
         String json = this.objectMapper.writeValueAsString(timeEntryDTO);
         String responseMessage = "startTime(" + timeEntryDTO.startTime() + ") " +
                 " | endTime (" + timeEntryDTO.endTime() + ")" +