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Helpers/GlobalExceptionMiddleware.cs

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+using Fitness_core_BE.Exceptions;
+using Fitness_core_BE.Models.IOs;
+
+namespace Fitness_core_BE.Helpers
+{
+    // The app has no central exception->HTTP mapping today -- an unhandled NotFoundException/
+    // BadRequestException/etc. (or the equivalent BCL KeyNotFoundException/ArgumentException
+    // many services throw) 500s with a raw stack trace instead of the ApiResponse shape every
+    // other error path returns. AgentController.cs already hand-rolls this exact mapping
+    // per-action (see its catch blocks); this middleware applies the same mapping once, for
+    // every controller, without having to add a try/catch to each action.
+    public class GlobalExceptionMiddleware
+    {
+        private readonly RequestDelegate _next;
+        private readonly ILogger<GlobalExceptionMiddleware> _logger;
+
+        public GlobalExceptionMiddleware(RequestDelegate next, ILogger<GlobalExceptionMiddleware> logger)
+        {
+            _next = next;
+            _logger = logger;
+        }
+
+        public async Task InvokeAsync(HttpContext context)
+        {
+            try
+            {
+                await _next(context);
+            }
+            catch (Exception ex)
+            {
+                var (statusCode, message) = ex switch
+                {
+                    NotFoundException or KeyNotFoundException => (404, ex.Message),
+                    BadRequestException or ArgumentException => (400, ex.Message),
+                    UnauthorizedException => (401, ex.Message),
+                    ForbiddenException => (403, ex.Message),
+                    ServiceUnavailableException => (503, ex.Message),
+                    _ => (500, "An unexpected error occurred."),
+                };
+
+                if (statusCode == 500)
+                    _logger.LogError(ex, "Unhandled exception for {Method} {Path}", context.Request.Method, context.Request.Path);
+
+                if (context.Response.HasStarted)
+                    throw;
+
+                context.Response.Clear();
+                context.Response.StatusCode = statusCode;
+                context.Response.ContentType = "application/json";
+                await context.Response.WriteAsJsonAsync(new ApiResponse<object>
+                {
+                    StatusCode = statusCode,
+                    Message = message,
+                    Data = null!
+                });
+            }
+        }
+    }
+}