refactor(configmanager): split into Core, CLI, and UI projects

Extract shared models, services, and application logic into
JdeScoping.ConfigManager.Core library. Add JdeScoping.ConfigManager.Cli
console app with validate, test-connection, and secret commands using
System.CommandLine. UI project now references Core for platform-agnostic
functionality while retaining Avalonia-specific dialog and clipboard services.

NEW/src/Utils/JdeScoping.ConfigManager.Core/Services/IConnectionTestService.cs

@@ -0,0 +1,18 @@
+using JdeScoping.ConfigManager.Core.Models;
+
+namespace JdeScoping.ConfigManager.Core.Services;
+
+/// <summary>
+/// Service for testing database connections.
+/// </summary>
+public interface IConnectionTestService
+{
+    /// <summary>
+    /// Tests a database connection asynchronously.
+    /// </summary>
+    /// <param name="connectionString">The connection string to test.</param>
+    /// <param name="provider">The database provider type.</param>
+    /// <param name="cancellationToken">Cancellation token for the operation.</param>
+    /// <returns>A ConnectionTestResult indicating success or failure of the test.</returns>
+    Task<ConnectionTestResult> TestConnectionAsync(string connectionString, ConnectionProvider provider, CancellationToken cancellationToken = default);
+}