Fixes #5453. Fix Windows VT input encoding for IME and non-ASCII characters

[BDisp]

Fixes

• Fixes # Fix Windows VT input encoding for IME and non-ASCII characters #5453

Proposed Changes/Todos

• Save the current console input code page.
• Set the console input code page to UTF-8 (65001).
• Decode VT input using UTF-8.
• Restore the original console input code page when disposing the driver.

Verified manually by entering Chinese IME characters (腌) before and after the change. Prior to the fix the character was received as ?; after the fix the correct Unicode character is delivered to Terminal.Gui.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[Copilot]

Pull request overview

Fixes Windows ANSI/VT driver input decoding on Windows when the console input code page is not UTF-8, improving IME and non-ASCII character handling (Fixes #5453).

Changes:

• Add SetConsoleCP(65001) support to force the console input code page to UTF-8 while using Windows VT input.
• Track and restore the original console input code page during driver teardown.