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/ExceptionHandlingMiddleware.cs

@@ -16,12 +16,21 @@ public class ExceptionHandlingMiddleware
     private readonly RequestDelegate _next;
     private readonly ILogger<ExceptionHandlingMiddleware> _logger;
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ExceptionHandlingMiddleware"/> class.
+    /// </summary>
+    /// <param name="next">The next middleware in the pipeline.</param>
+    /// <param name="logger">The logger instance for exception logging.</param>
     public ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
     {
         _next = next;
         _logger = logger;
     }
 
+    /// <summary>
+    /// Invokes the middleware to handle incoming HTTP requests and catch exceptions.
+    /// </summary>
+    /// <param name="context">The HTTP context for the current request.</param>
     public async Task InvokeAsync(HttpContext context)
     {
         try

DocumentOperator.API/Middleware/RequestLoggingMiddleware.cs

@@ -1,5 +1,8 @@
 namespace DocumentOperator.API.Middleware
 {
+    /// <summary>
+    /// Placeholder middleware for HTTP request/response logging.
+    /// </summary>
     public class RequestLoggingMiddleware
     {
     }

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
     {
     }