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/Middleware/TenantResolutionMiddleware.cs

@@ -1,5 +1,8 @@
 namespace DocumentOperator.API.Middleware
 {
+    /// <summary>
+    /// Placeholder middleware for multi-tenancy resolution via X-API-Key header.
+    /// </summary>
     public class TenantResolutionMiddleware
     {
     }