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/OfflineModeGuard.cs

@@ -0,0 +1,41 @@
+using System;
+
+namespace StellaOps.Cli.Services;
+
+/// <summary>
+/// Guard for operations that require network connectivity.
+/// Stub implementation - will be wired to actual offline mode detection.
+/// </summary>
+internal static class OfflineModeGuard
+{
+    /// <summary>
+    /// Gets whether the CLI is currently in offline mode.
+    /// </summary>
+    public static bool IsOffline { get; set; }
+
+    /// <summary>
+    /// Gets whether network operations are allowed.
+    /// </summary>
+    public static bool IsNetworkAllowed() => !IsOffline;
+
+    /// <summary>
+    /// Gets whether network operations are allowed, checking options and logging operation.
+    /// </summary>
+    /// <param name="options">CLI options (used to check offline mode setting).</param>
+    /// <param name="operationName">Name of the operation being checked.</param>
+    public static bool IsNetworkAllowed(object? options, string operationName) => !IsOffline;
+
+    /// <summary>
+    /// Throws if the CLI is in offline mode and the operation requires network.
+    /// </summary>
+    /// <param name="operationName">Name of the operation being guarded.</param>
+    /// <exception cref="InvalidOperationException">Thrown when offline and network required.</exception>
+    public static void ThrowIfOffline(string operationName)
+    {
+        if (IsOffline)
+        {
+            throw new InvalidOperationException(
+                $"Operation '{operationName}' requires network connectivity but CLI is in offline mode.");
+        }
+    }
+}