fix(auth): canonical username from the entry; refuse empty passwords in both bind modes

- Request UserNameAttribute in the search (de-duplicated, case-insensitive) and return its
  first value as LdapAuthResult.Username, falling back to the typed name (I2).
- The empty-password guard now applies in ServiceAccount mode too: AD treats an empty-password
  simple bind as an unauthenticated bind and reports success (I3).
- Restore the dropped explanatory comments in CompleteWithServiceAccount / SearchUser.

ZB.MOM.WW.Auth/tests/ZB.MOM.WW.Auth.Ldap.Tests/LdapAuthServiceUserBindTests.cs

@@ -178,4 +178,45 @@ public class LdapAuthServiceUserBindTests
 
         Assert.Equal(LdapAuthFailure.GroupLookupFailed, result.Failure);
     }
+
+    [Fact]
+    public async Task Username_IsCanonicalisedFromTheEntry_NotTheTypedCase()
+    {
+        var factory = new SequenceLdapConnectionFactory(new FakeLdapConnection().WithUserEntry(
+            "CN=Jane Doe,OU=u,DC=x", new[] { "CN=UG_A,OU=g,DC=x" }, "Jane Doe",
+            new Dictionary<string, string[]> { ["sAMAccountName"] = new[] { "jdoe" } }));
+        var result = await Login(new LdapAuthService(Opts(), factory), @"NAM\JDOE");
+
+        Assert.True(result.Succeeded);
+        Assert.Equal("jdoe", result.Username);
+    }
+
+    [Fact]
+    public async Task Username_FallsBackToTheTypedName_WhenTheEntryLacksTheAttribute()
+    {
+        var factory = new SequenceLdapConnectionFactory(Jane());
+        var result = await Login(new LdapAuthService(Opts(), factory), "JDOE");
+
+        Assert.True(result.Succeeded);
+        Assert.Equal("JDOE", result.Username);
+    }
+
+    [Fact]
+    public async Task Search_RequestsTheUserNameAttribute()
+    {
+        var factory = new SequenceLdapConnectionFactory(Jane());
+        await Login(new LdapAuthService(Opts(), factory), "jdoe");
+
+        Assert.Equal(new[] { "displayName", "memberOf", "sAMAccountName" }, factory.Created[0].SearchAttributes);
+    }
+
+    [Fact]
+    public async Task Search_DeDuplicatesTheUserNameAttribute_CaseInsensitively()
+    {
+        var factory = new SequenceLdapConnectionFactory(Jane());
+        var svc = new LdapAuthService(Opts() with { DisplayNameAttribute = "SAMACCOUNTNAME" }, factory);
+        await Login(svc, "jdoe");
+
+        Assert.Equal(new[] { "SAMACCOUNTNAME", "memberOf" }, factory.Created[0].SearchAttributes);
+    }
 }