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/Output/CliError.cs

@@ -121,6 +121,19 @@ public sealed record CliError(
         return 1;
     }
 
+    /// <summary>
+    /// Error code for offline mode violations.
+    /// </summary>
+    public const string OfflineMode = "ERR_OFFLINE_MODE";
+
+    /// <summary>
+    /// Creates an error from an error code.
+    /// </summary>
+    public static CliError FromCode(string code, string? message = null)
+    {
+        return new CliError(code, message ?? $"Error: {code}");
+    }
+
     /// <summary>
     /// Creates an error from an exception.
     /// </summary>
@@ -167,7 +180,7 @@ public sealed record CliError(
     /// Creates an error from a parsed API error.
     /// CLI-SDK-62-002: Surfaces standardized API error envelope fields.
     /// </summary>
-    public static CliError FromParsedApiError(ParsedApiError error)
+    internal static CliError FromParsedApiError(ParsedApiError error)
     {
         ArgumentNullException.ThrowIfNull(error);
 
@@ -195,7 +208,7 @@ public sealed record CliError(
     /// Creates an error from an API error envelope.
     /// CLI-SDK-62-002: Direct conversion from envelope format.
     /// </summary>
-    public static CliError FromApiErrorEnvelope(ApiErrorEnvelope envelope, int httpStatus)
+    internal static CliError FromApiErrorEnvelope(ApiErrorEnvelope envelope, int httpStatus)
     {
         ArgumentNullException.ThrowIfNull(envelope);