docs: Add XML documentation comments to API layer

Add missing XML doc comments to resolve CS1591 warnings:

  - SerilogConfiguration: Class comment

  - SwaggerConfiguration: Class and AddSwaggerDocumentation() method

  - ExceptionHandlingMiddleware: Constructor and InvokeAsync() method

  - RequestLoggingMiddleware: Placeholder class comment

  - TenantResolutionMiddleware: Placeholder class comment

  - Program: Partial class comment for integration test access

Result: 0 CS1591 warnings in DocumentOperator.API project

DocumentOperator.API/Configuration/SwaggerConfiguration.cs

@@ -3,8 +3,16 @@ using System.Reflection;
 
 namespace DocumentOperator.API.Configuration
 {
+    /// <summary>
+    /// Provides extension methods for configuring Swagger/OpenAPI documentation.
+    /// </summary>
     public static class SwaggerConfiguration
     {
+        /// <summary>
+        /// Adds Swagger documentation generation to the service collection.
+        /// </summary>
+        /// <param name="services">The service collection to add Swagger to.</param>
+        /// <returns>The modified service collection.</returns>
         public static IServiceCollection AddSwaggerDocumentation(this IServiceCollection services)
         {
             services.AddSwaggerGen(options =>