Fix LDAP referral callback host name marshaling

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Description

Referral callbacks decoded the native PCHAR HostName as UTF-16, producing garbled referral server identifiers.

• Windows callback ABI
  • Decode HostName with Marshal.PtrToStringAnsi in both query and notify callback paths.
  • Preserve UTF-16 decoding for NewDN.

string target = $"{Marshal.PtrToStringAnsi(hostNamePtr)}:{portNumber}";

• Regression coverage
  • Add Windows-focused tests for both callback paths using ANSI host-name buffers.

Customer Impact

Referral callback consumers receive the correct referred server identifier when chasing referrals.

Regression

Behavior differs from .NET Framework; this corrects the Windows callback marshaling mismatch.

Testing

Focused callback marshaling coverage for query and notify callbacks.

Risk

Low. The change aligns pointer decoding with the documented Windows LDAP callback ABI.

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• Fixes LdapConnection incorrectly marshal HostName string for ReferralCallback #121439

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Review details

Suppressed comments (4)

src/libraries/System.DirectoryServices.Protocols/tests/ReferralCallbackTests.cs:70

• Same concern as the QueryForConnection marshaling test: this uses reflection over non-public members (including the private _ldapHandle field) and should be skipped on .NET Framework to avoid TFM-specific failures.

        [Fact]
        [PlatformSpecific(TestPlatforms.Windows)]
        public void ProcessNotifyConnection_MarshalsHostNameAsAnsi()

src/libraries/System.DirectoryServices.Protocols/tests/ReferralCallbackTests.cs:45

• This test project multi-targets .NET Framework, but this test uses reflection over non-public implementation details (e.g., LdapSessionOptions.ProcessQueryConnection / LdapConnection._ldapHandle) that are not stable across TFMs. Other tests in this project explicitly skip .NET Framework for similar reflection-based access; this one should too to avoid NetFx-only failures.

This issue also appears on line 68 of the same file.

        [Fact]
        [PlatformSpecific(TestPlatforms.Windows)]
        public void ProcessQueryConnection_MarshalsHostNameAsAnsi()

src/libraries/System.DirectoryServices.Protocols/tests/ReferralCallbackTests.cs:166

• AllocateAnsiString manually builds an ASCII buffer and double-null terminates it. Using Marshal.StringToHGlobalAnsi better matches the code under test (PtrToStringAnsi uses the active ANSI code page), reduces test-specific encoding assumptions, and simplifies the allocation logic.

        private static IntPtr AllocateAnsiString(string value)
        {
            byte[] bytes = System.Text.Encoding.ASCII.GetBytes(value);
            IntPtr pointer = Marshal.AllocHGlobal(bytes.Length + 2);
            Marshal.Copy(bytes, 0, pointer, bytes.Length);

src/libraries/System.DirectoryServices.Protocols/tests/ReferralCallbackTests.cs:169

• GetCallbackMethod currently returns null if the method name/signature changes, which then fails later as a NullReferenceException. Throwing a targeted exception here makes failures easier to diagnose (especially given this is reflection over a private method).

        private static MethodInfo GetCallbackMethod(string name) =>
            typeof(LdapSessionOptions).GetMethod(name, BindingFlags.Instance | BindingFlags.NonPublic);

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

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