Add integration tests for migration categories and execution

- Implemented MigrationCategoryTests to validate migration categorization for startup, release, seed, and data migrations.
- Added tests for edge cases, including null, empty, and whitespace migration names.
- Created StartupMigrationHostTests to verify the behavior of the migration host with real PostgreSQL instances using Testcontainers.
- Included tests for migration execution, schema creation, and handling of pending release migrations.
- Added SQL migration files for testing: creating a test table, adding a column, a release migration, and seeding data.

src/Cli/StellaOps.Cli/Services/BackendOperationsClient.cs

@@ -28,6 +28,7 @@ namespace StellaOps.Cli.Services;
 internal sealed class BackendOperationsClient : IBackendOperationsClient
 {
     private static readonly JsonSerializerOptions SerializerOptions = new(JsonSerializerDefaults.Web);
+    private static readonly JsonSerializerOptions JsonOptions = SerializerOptions;
     private static readonly TimeSpan TokenRefreshSkew = TimeSpan.FromSeconds(30);
     private static readonly IReadOnlyDictionary<string, object?> EmptyMetadata =
         new ReadOnlyDictionary<string, object?>(new Dictionary<string, object?>(0, StringComparer.OrdinalIgnoreCase));
@@ -523,8 +524,7 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
         using var response = await _httpClient.SendAsync(httpRequest, cancellationToken).ConfigureAwait(false);
         if (!response.IsSuccessStatusCode)
         {
-            var (message, problem) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
-            var errorCode = ExtractProblemErrorCode(problem);
+            var (message, errorCode) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
             throw new PolicyApiException(message, response.StatusCode, errorCode);
         }
 
@@ -639,8 +639,7 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
         using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
         if (!response.IsSuccessStatusCode)
         {
-            var (message, problem) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
-            var errorCode = ExtractProblemErrorCode(problem);
+            var (message, errorCode) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
             throw new PolicyApiException(message, response.StatusCode, errorCode);
         }
 
@@ -758,8 +757,7 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
         using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
         if (!response.IsSuccessStatusCode)
         {
-            var (message, problem) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
-            var errorCode = ExtractProblemErrorCode(problem);
+            var (message, errorCode) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
             throw new PolicyApiException(message, response.StatusCode, errorCode);
         }
 
@@ -807,8 +805,7 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
         using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
         if (!response.IsSuccessStatusCode)
         {
-            var (message, problem) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
-            var errorCode = ExtractProblemErrorCode(problem);
+            var (message, errorCode) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
             throw new PolicyApiException(message, response.StatusCode, errorCode);
         }
 
@@ -858,8 +855,7 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
         using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
         if (!response.IsSuccessStatusCode)
         {
-            var (message, problem) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
-            var errorCode = ExtractProblemErrorCode(problem);
+            var (message, errorCode) = await CreateFailureDetailsAsync(response, cancellationToken).ConfigureAwait(false);
             throw new PolicyApiException(message, response.StatusCode, errorCode);
         }
 
@@ -909,11 +905,11 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
             throw new InvalidOperationException(failure);
         }
 
-        var result = await response.Content.ReadFromJsonAsync<EntryTraceResponseModel>(SerializerOptions, cancellationToken).ConfigureAwait(false);
-        if (result is null)
-        {
-            throw new InvalidOperationException("EntryTrace response payload was empty.");
-        }
+        var result = await response.Content.ReadFromJsonAsync<EntryTraceResponseModel>(SerializerOptions, cancellationToken).ConfigureAwait(false);
+        if (result is null)
+        {
+            throw new InvalidOperationException("EntryTrace response payload was empty.");
+        }
 
         return result;
     }
@@ -4512,11 +4508,11 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
     }
 
     // CLI-SDK-64-001: SDK update operations
-    public async Task<SdkUpdateResponse> CheckSdkUpdatesAsync(SdkUpdateRequest request, CancellationToken cancellationToken)
-    {
-        ArgumentNullException.ThrowIfNull(request);
-
-        EnsureBackendConfigured();
+    public async Task<SdkUpdateResponse> CheckSdkUpdatesAsync(SdkUpdateRequest request, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        EnsureBackendConfigured();
         OfflineModeGuard.ThrowIfOffline("sdk update");
 
         var queryParams = new List<string>();
@@ -4554,9 +4550,9 @@ internal sealed class BackendOperationsClient : IBackendOperationsClient
             };
         }
 
-        var result = await response.Content.ReadFromJsonAsync<SdkUpdateResponse>(JsonOptions, cancellationToken).ConfigureAwait(false);
-        return result ?? new SdkUpdateResponse { Success = false, Error = "Empty response" };
-    }
+        var result = await response.Content.ReadFromJsonAsync<SdkUpdateResponse>(JsonOptions, cancellationToken).ConfigureAwait(false);
+        return result ?? new SdkUpdateResponse { Success = false, Error = "Empty response" };
+    }
 
     public async Task<SdkListResponse> ListInstalledSdksAsync(string? language, string? tenant, CancellationToken cancellationToken)
     {