fix(backend): Fix integration test failures - GlobalExceptionHandler and test isolation

Fixed 8 failing integration tests by addressing two root causes:

1. GlobalExceptionHandler returning incorrect HTTP status codes
   - Added handling for UnauthorizedAccessException → 401
   - Added handling for ArgumentException/InvalidOperationException → 400
   - Added handling for DbUpdateException (duplicate key) → 409
   - Now correctly maps exception types to HTTP status codes

2. Test isolation issue with shared HttpClient
   - Modified DatabaseFixture to create new HttpClient for each test
   - Prevents Authorization header pollution between tests
   - Ensures clean test state for authentication tests

Test Results:
- Before: 23/31 passed (8 failed)
- After: 31/31 passed (0 failed)

Changes:
- Enhanced GlobalExceptionHandler with proper status code mapping
- Fixed DatabaseFixture.Client to create isolated instances
- All authentication and RBAC tests now pass

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

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

colaflow-api/tests/Modules/Identity/ColaFlow.Modules.Identity.IntegrationTests/Infrastructure/DatabaseFixture.cs

@@ -8,18 +8,28 @@ namespace ColaFlow.Modules.Identity.IntegrationTests.Infrastructure;
 public class DatabaseFixture : IDisposable
 {
     public ColaFlowWebApplicationFactory Factory { get; }
-    public HttpClient Client { get; }
+
+    // Note: Client property is kept for backward compatibility but creates new instances
+    // Tests should call CreateClient() for isolation to avoid shared state issues
+    public HttpClient Client => CreateClient();
 
     public DatabaseFixture()
     {
         // Use In-Memory Database for fast, isolated tests
         Factory = new ColaFlowWebApplicationFactory(useInMemoryDatabase: true);
-        Client = Factory.CreateClient();
+    }
+
+    /// <summary>
+    /// Creates a new HttpClient for each test to ensure test isolation
+    /// Prevents Authorization header sharing between tests
+    /// </summary>
+    public HttpClient CreateClient()
+    {
+        return Factory.CreateClient();
     }
 
     public void Dispose()
     {
-        Client?.Dispose();
         Factory?.Dispose();
         GC.SuppressFinalize(this);
     }