Package Manager Console corrupts supplementary Unicode characters on backspace

[bribrothers]

Description

Visual Studio Package Manager Console handles backspace as a single UTF-16 code unit. For supplementary-plane characters represented by surrogate pairs, backspace can delete only the low surrogate and leave a corrupt unpaired high surrogate in both the console buffer and returned input. This surfaced during VS 2026 GB18030-2022 certification testing with CJK Extension B and supplementary private-use characters.

Proposed fix

• Update WpfConsole.WriteBackspace to remove a complete trailing surrogate pair.
• Update NuGetHostUserInterface.ReadLine to remove the same complete pair from its input buffer.
• Add an Apex regression test that echoes a mixed supplementary-plane sample through Package Manager Console.

Tracking

• Azure DevOps bug: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/3042229
• Agency prototype PR: https://dev.azure.com/devdiv/DevDiv/_git/NuGet-NuGet.Client-Trusted/pullrequest/766174

The Agency prototype passed its remote build and test validation.