fix(scripting/mono-rt2): Fix UTF8 encoding issues for surrogate pairs and Korean characters

[poco8537]

Goal of this PR

Ensure that UTF8 encoding in the Mono v2 scripting environment correctly handles surrogate pairs and Korean characters, preventing string corruption during native interop.

How is this PR achieving the goal

This PR updates the UTF8 encoding functions to correctly handle surrogate pairs by encoding valid pairs as 4-byte sequences and replacing invalid or missing surrogates with the replacement character (U+FFFD), ensuring that Korean and other Unicode characters are encoded properly.

This PR applies to the following area(s)

ScRT: C#

Successfully tested on

Game builds: ..

Platforms: Windows, Linux

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

Fixes issues

fixs #3263