fix: tighten MxGateway Ldap:Port to 1-65535; catch IOException in path validation

Defect 1: ValidateLdap used AddIfNotPositive for Port, accepting any value
> 0 including 70000. Replaced with builder.Port() from the shared
ZB.MOM.WW.Configuration library, which enforces the 1-65535 TCP range and
emits "MxGateway:Ldap:Port must be between 1 and 65535 (was {value})".

Defect 2: AddIfInvalidPath only caught ArgumentException, NotSupportedException,
and PathTooLongException from Path.GetFullPath. On macOS/Linux a path containing
an embedded null throws IOException, which escaped the catch block and caused
Validate() to throw instead of returning a failure. Added catch (IOException).

Tests: added Validate_Fails_WhenLdapPortIsZero, Validate_Fails_WhenLdapPortExceedsMaximum,
and Validate_Succeeds_WhenLdapEnabledWithValidPort to cover the new range boundary.

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -80,7 +80,7 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             options.GroupAttribute,
             "MxGateway:Ldap:GroupAttribute is required when LDAP login is enabled.",
             builder);
-        AddIfNotPositive(options.Port, "MxGateway:Ldap:Port must be greater than zero.", builder);
+        builder.Port(options.Port, "MxGateway:Ldap:Port");
 
         if (!options.UseTls && !options.AllowInsecureLdap)
         {
@@ -339,5 +339,9 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
         {
             builder.Add(message);
         }
+        catch (IOException)
+        {
+            builder.Add(message);
+        }
     }
 }

src/ZB.MOM.WW.MxGateway.Tests/Configuration/GatewayOptionsValidatorTests.cs

@@ -9,6 +9,29 @@ public sealed class GatewayOptionsValidatorTests
     // design-default values; those defaults are validated separately in GatewayOptionsTests.
     private static GatewayOptions ValidOptions() => new();
 
+    // Returns enabled LDAP options that pass all checks except Port.
+    // The class defaults already satisfy the blank-field checks; we only
+    // override Enabled (must be true to exercise the port check) and Port.
+    private static LdapOptions LdapOptionsWithPort(int port) => new()
+    {
+        Enabled = true,
+        Port = port,
+    };
+
+    private static GatewayOptions CloneWithLdap(GatewayOptions source, LdapOptions ldap)
+        => new()
+        {
+            Authentication = source.Authentication,
+            Ldap = ldap,
+            Worker = source.Worker,
+            Sessions = source.Sessions,
+            Events = source.Events,
+            Dashboard = source.Dashboard,
+            Protocol = source.Protocol,
+            Alarms = source.Alarms,
+            Tls = source.Tls,
+        };
+
     private static GatewayOptions CloneWithTls(GatewayOptions source, TlsOptions tls)
         => new()
         {
@@ -65,4 +88,34 @@ public sealed class GatewayOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains(result.Failures!, f => f.Contains("MxGateway:Tls:SelfSignedCertPath must not be blank."));
     }
+
+    [Fact]
+    public void Validate_Fails_WhenLdapPortIsZero()
+    {
+        GatewayOptions options = CloneWithLdap(ValidOptions(), LdapOptionsWithPort(0));
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(
+            result.Failures!,
+            f => f.Contains("MxGateway:Ldap:Port must be between 1 and 65535 (was 0)"));
+    }
+
+    [Fact]
+    public void Validate_Fails_WhenLdapPortExceedsMaximum()
+    {
+        GatewayOptions options = CloneWithLdap(ValidOptions(), LdapOptionsWithPort(70000));
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(
+            result.Failures!,
+            f => f.Contains("MxGateway:Ldap:Port must be between 1 and 65535 (was 70000)"));
+    }
+
+    [Fact]
+    public void Validate_Succeeds_WhenLdapEnabledWithValidPort()
+    {
+        GatewayOptions options = CloneWithLdap(ValidOptions(), LdapOptionsWithPort(389));
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Succeeded);
+    }
 }