feat: Use IPNetwork.TryParse() for CIDR parsing (#1122)

## Motivation

The CIDR parsing logic was implemented with a custom helper that
manually split strings, parsed integers, and validated prefix lengths.
The .NET framework provides a built-in and thoroughly validated
`IPNetwork.TryParse()` API that handles all of this, eliminating the
need for custom parsing code.

## Changes

- Removed the `TryParseCidr()` wrapper method entirely
- Inlined CIDR validation directly in the loop using
`IPNetwork.TryParse()`
- Removed unused `System.Net.Sockets` import

This eliminates unnecessary indirection while maintaining identical
behavior and improving maintainability by relying on framework-tested
APIs.

Author: BenjaminMichaelis

EssentialCSharp.Web/Extensions/IServiceCollectionExtensions.cs

@@ -1,5 +1,4 @@
 using System.Net;
-using System.Net.Sockets;
 using EssentialCSharp.Web.Services;
 using Microsoft.AspNetCore.HttpOverrides;
 
@@ -48,7 +47,7 @@ public static void AddTrustedForwardedHeaders(this IServiceCollection services,
 
             foreach (var cidr in trustedProxyCidrs)
             {
-                if (!TryParseCidr(cidr, out var network))
+                if (string.IsNullOrWhiteSpace(cidr) || !System.Net.IPNetwork.TryParse(cidr.Trim(), out var network))
                     throw new InvalidOperationException($"Invalid ForwardedHeaders:TrustedProxyCidrs entry '{cidr}'. Use CIDR notation, e.g. '10.0.0.0/8'.");
 
                 options.KnownIPNetworks.Add(network);
@@ -63,30 +62,4 @@ public static void AddTrustedForwardedHeaders(this IServiceCollection services,
             }
         });
     }
-
-    private static bool TryParseCidr(string cidr, out System.Net.IPNetwork network)
-    {
-        network = default!;
-        if (string.IsNullOrWhiteSpace(cidr))
-            return false;
-
-        string[] parts = cidr.Split('/', 2, StringSplitOptions.TrimEntries);
-        if (parts.Length != 2
-            || !IPAddress.TryParse(parts[0], out var networkAddress)
-            || !int.TryParse(parts[1], out var prefixLength))
-            return false;
-
-        int maxPrefixLength = networkAddress.AddressFamily switch
-        {
-            AddressFamily.InterNetwork => 32,
-            AddressFamily.InterNetworkV6 => 128,
-            _ => -1
-        };
-
-        if (maxPrefixLength < 0 || prefixLength < 0 || prefixLength > maxPrefixLength)
-            return false;
-
-        network = new System.Net.IPNetwork(networkAddress, prefixLength);
-        return true;
-    }
 }