test(backend): Fix all compilation errors in Domain.Tests after Day 15 multi-tenant changes

Fixed 73 compilation errors caused by Epic/Story/WorkTask.Create() now requiring TenantId parameter.

Changes:
- Created TestDataBuilder helper class for test data creation
- Updated EpicTests.cs: Added TenantId to all Epic.Create() calls (10 tests)
- Updated StoryTests.cs: Added TenantId to all Story.Create() calls (26 tests)
- Updated WorkTaskTests.cs: Added TenantId to all WorkTask.Create() calls (37 tests)
- Added using ColaFlow.Domain.Tests.Builders to all test files

Test Results:
- Build: 0 errors, 9 warnings (xUnit analyzer warnings only)
- Tests: 192/192 passed in ColaFlow.Domain.Tests
- Full Suite: 427/431 passed (4 skipped as expected)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

colaflow-api/tests/ColaFlow.Domain.Tests/Builders/TestDataBuilder.cs

@@ -0,0 +1,99 @@
+using ColaFlow.Modules.ProjectManagement.Domain.Aggregates.ProjectAggregate;
+using ColaFlow.Modules.ProjectManagement.Domain.ValueObjects;
+
+namespace ColaFlow.Domain.Tests.Builders;
+
+/// <summary>
+/// Test Data Builder for creating test entities with default values.
+/// Simplifies test setup by providing consistent test data.
+/// </summary>
+public static class TestDataBuilder
+{
+    /// <summary>
+    /// Creates a test TenantId with a random GUID.
+    /// </summary>
+    public static TenantId CreateTestTenantId()
+        => TenantId.From(Guid.NewGuid());
+
+    /// <summary>
+    /// Creates a test ProjectId with a random GUID.
+    /// </summary>
+    public static ProjectId CreateTestProjectId()
+        => ProjectId.From(Guid.NewGuid());
+
+    /// <summary>
+    /// Creates a test UserId with a random GUID.
+    /// </summary>
+    public static UserId CreateTestUserId()
+        => UserId.From(Guid.NewGuid());
+
+    /// <summary>
+    /// Creates a test EpicId with a random GUID.
+    /// </summary>
+    public static EpicId CreateTestEpicId()
+        => EpicId.Create();
+
+    /// <summary>
+    /// Creates a test StoryId with a random GUID.
+    /// </summary>
+    public static StoryId CreateTestStoryId()
+        => StoryId.Create();
+
+    /// <summary>
+    /// Creates a test Epic with default or provided values.
+    /// </summary>
+    public static Epic CreateTestEpic(
+        TenantId? tenantId = null,
+        string name = "Test Epic",
+        string description = "Test Epic Description",
+        ProjectId? projectId = null,
+        UserId? createdBy = null)
+    {
+        return Epic.Create(
+            tenantId ?? CreateTestTenantId(),
+            name,
+            description,
+            projectId ?? CreateTestProjectId(),
+            createdBy ?? CreateTestUserId());
+    }
+
+    /// <summary>
+    /// Creates a test Story with default or provided values.
+    /// </summary>
+    public static Story CreateTestStory(
+        TenantId? tenantId = null,
+        string title = "Test Story",
+        string description = "Test Story Description",
+        EpicId? epicId = null,
+        TaskPriority? priority = null,
+        UserId? createdBy = null)
+    {
+        return Story.Create(
+            tenantId ?? CreateTestTenantId(),
+            title,
+            description,
+            epicId ?? CreateTestEpicId(),
+            priority ?? TaskPriority.Medium,
+            createdBy ?? CreateTestUserId());
+    }
+
+    /// <summary>
+    /// Creates a test WorkTask with default or provided values.
+    /// </summary>
+    public static WorkTask CreateTestTask(
+        TenantId? tenantId = null,
+        string title = "Test Task",
+        string description = "Test Task Description",
+        StoryId? storyId = null,
+        TaskPriority? priority = null,
+        UserId? createdBy = null)
+    {
+        return WorkTask.Create(
+            tenantId ?? CreateTestTenantId(),
+            title,
+            description,
+            storyId ?? CreateTestStoryId(),
+            priority ?? TaskPriority.Medium,
+            createdBy ?? CreateTestUserId());
+    }
+}