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Prefab/Endpoints/ApiProblemDetails.cs

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+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Prefab.Endpoints;
+
+/// <summary>
+/// Shared helpers for building consistent HTTP problem responses across modules.
+/// </summary>
+public static class ApiProblemDetails
+{
+    /// <summary>
+    /// Creates a 404 <see cref="ProblemDetails"/> payload with standard metadata and resource identifiers.
+    /// </summary>
+    /// <param name="resource">Logical name of the missing resource (for example, "Product").</param>
+    /// <param name="identifier">Identifier value supplied by the caller.</param>
+    /// <param name="detail">Human-readable explanation for logging or the client.</param>
+    public static ProblemDetails NotFound(string resource, string identifier, string detail) =>
+        new()
+        {
+            Title = "Resource not found.",
+            Detail = detail,
+            Type = "https://prefab.dev/problems/not-found",
+            Status = StatusCodes.Status404NotFound,
+            Extensions =
+            {
+                ["resource"] = resource,
+                ["identifier"] = identifier
+            }
+        };
+
+    /// <summary>
+    /// Creates a 409 <see cref="ProblemDetails"/> payload used when a request cannot be completed due to a resource conflict.
+    /// </summary>
+    /// <param name="detail">Human-readable explanation of the conflict.</param>
+    /// <param name="resource">Logical name of the resource involved, when known.</param>
+    /// <param name="identifier">Identifier value supplied by the caller, when known.</param>
+    public static ProblemDetails Conflict(string detail, string? resource = null, string? identifier = null)
+    {
+        var problem = new ProblemDetails
+        {
+            Title = "Request conflict.",
+            Detail = detail,
+            Type = "https://prefab.dev/problems/conflict",
+            Status = StatusCodes.Status409Conflict
+        };
+
+        if (!string.IsNullOrWhiteSpace(resource))
+        {
+            problem.Extensions["resource"] = resource;
+        }
+
+        if (!string.IsNullOrWhiteSpace(identifier))
+        {
+            problem.Extensions["identifier"] = identifier;
+        }
+
+        return problem;
+    }
+}