Propagate ServiceResult metadata as HTTP response headers

Metadata set via WithMetadata() is now written as HTTP headers
through ApiResponseBuilder.ApplyHeaders(). Same pattern as the
earlier Errors propagation fix.

Author: imclint21

src/Initium/Infrastructure/Filters/ApiResponseFilter.cs

@@ -45,16 +45,25 @@ public override void OnActionExecuted(ActionExecutedContext context)
         if (serviceResult.Errors != null)
 	        apiResponseBuilder.WithErrors(serviceResult.Errors.ToArray());
 
+        // Propagate metadata from the ServiceResult as HTTP response headers.
+        if (serviceResult.Metadata.Count > 0)
+	        apiResponseBuilder.WithCustomHeaders(serviceResult.Metadata);
+
+        // Build the final response with status code and message.
+        apiResponseBuilder
+	        .WithStatusCode(statusCode)
+	        .WithMessage(message);
+
+        // Write custom headers (from metadata and attributes) to the HTTP response.
+        apiResponseBuilder.ApplyHeaders(context.HttpContext);
+
         // Determine the appropriate response based on the status code:
         // - For 204 (No Content) and 304 (Not Modified), set a StatusCodeResult without a response body.
         // - For other status codes, construct a standardized ApiResponse object including HTTP context details.
         context.Result = statusCode switch
         {
 	        HttpStatusCode.NoContent or HttpStatusCode.NotModified => new StatusCodeResult((int)statusCode),
-	        _ => apiResponseBuilder
-		        .WithStatusCode(statusCode)
-		        .WithMessage(message)
-		        .BuildAsJsonResult()
+	        _ => apiResponseBuilder.BuildAsJsonResult()
         };
     }
 }

src/Initium/Response/ApiResponseBuilder.cs

@@ -125,8 +125,25 @@ public ApiResponseBuilder WithCustomHeaders(Dictionary<string, string> headers)
     /// Builds and returns the constructed <see cref="Response.ApiResponse"/> as a <see cref="JsonResult"/>.
     /// </summary>
     /// <returns>A <see cref="JsonResult"/> containing the constructed <see cref="Response.ApiResponse"/>.</returns>
-    public JsonResult BuildAsJsonResult() => new(ApiResponse)
+    public JsonResult BuildAsJsonResult()
     {
-        StatusCode = ApiResponse.StatusCode
-    };
+        var result = new JsonResult(ApiResponse)
+        {
+            StatusCode = ApiResponse.StatusCode
+        };
+
+        return result;
+    }
+
+    /// <summary>
+    /// Writes any custom headers from the <see cref="ApiResponse"/> into the HTTP response.
+    /// Should be called before the result is executed.
+    /// </summary>
+    public void ApplyHeaders(Microsoft.AspNetCore.Http.HttpContext context)
+    {
+        if (ApiResponse.CustomHeaders == null) return;
+
+        foreach (var header in ApiResponse.CustomHeaders)
+            context.Response.Headers[header.Key] = header.Value;
+    }
 }