This PR systematically replaces direct calls to Marshal.PtrToStringUni and Marshal.StringToCoTaskMemUni with equivalent methods from Utf16StringMarshaller throughout the codebase, improving type consistency and centralizing UTF-16 string marshalling logic.

Changes Made

• Replaced Marshal method calls: Updated approximately 69 occurrences across 15+ core library files to use Utf16StringMarshaller.ConvertToManaged() and Utf16StringMarshaller.ConvertToUnmanaged()
• Updated type signatures: Changed method signatures and struct fields from IntPtr/char* to ushort* for UTF-16 string pointers, eliminating unnecessary casts
• Added unsafe contexts: Added unsafe modifiers to methods and structs where needed for unmanaged pointer usage
• Enhanced interop consistency: Updated interop structures (NCRYPT_UI_POLICY, CERT_CHAIN_ELEMENT, UNICODE_INTPTR_STRING) to use ushort* for string fields

Key Files Updated

• System.Private.CoreLib: CultureData.Browser.cs, EventPipe.cs, NetworkCredential.cs
• System.Security.Cryptography: CngKey.StandardProperties.cs, X509 certificate handling files, Windows interop structures
• System.DirectoryServices.AccountManagement: SidList.cs, AuthZSet.cs with LSA structure updates
• System.Runtime.InteropServices.JavaScript: JSFunctionBinding.cs, JSMarshalerArgument.String.cs
• Microsoft.Win32.SystemEvents: SystemEvents.cs
• System.Data.Odbc: OdbcStatementHandle.cs, DbBuffer.cs
• System.Security.AccessControl: Win32.cs

Pattern Examples

Before:

string result = Marshal.PtrToStringUni(ptr);
IntPtr nativeStr = Marshal.StringToCoTaskMemUni(managed);
Marshal.FreeCoTaskMem(nativeStr);

After:

string result = Utf16StringMarshaller.ConvertToManaged((ushort*)ptr);
ushort* nativeStr = Utf16StringMarshaller.ConvertToUnmanaged(managed);
Utf16StringMarshaller.Free(nativeStr);

For cases requiring length specification, the string constructor is used:

// Before: Marshal.PtrToStringUni(buffer, length)
// After: new string((char*)buffer, 0, length)

All changes maintain backward compatibility while improving type safety and consistency. Interop tests were excluded from modifications as specified. The original Utf16StringMarshaller.cs implementation remains unchanged, serving as the centralized marshalling implementation.

Fixes #118988.

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