Fix UTF-16 TextDecoder unpaired surrogate handling (#4612)

[Rahul-2k4]

Summary

• make UTF-16 TextDecoder output well-formed strings by replacing unpaired surrogates with U+FFFD
• apply the fix to both utf-16le and utf-16be decode paths
• keep truncation behavior correct for odd-length input (no extra replacement when the last decoded code unit is a high surrogate)

Tests

• add regression tests for unpaired surrogate replacement in utf-16le and utf-16be
• add regression tests for truncation edge cases in both endian modes
• verified with:
  • cargo test -p boa_runtime decoder_utf16
  • cargo test -p boa_runtime
  • cargo fmt --all --check
  • cargo clippy -p boa_runtime --all-targets -- -D warnings

Closes #4612.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,504	49,504	0
Ignored	2,262	2,262	0
Failed	1,096	1,096	0
Panics	0	0	0
Conformance	93.65%	93.65%	0.00%

[zhuzhu81998]

@Rahul-2k4 are you still working on this?

[Rahul-2k4]

yep just finished in a while rn final commits are taking place

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.13%. Comparing base (6ddc2b4) to head (45fcd6e).
⚠️ Report is 679 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4650      +/-   ##
==========================================
+ Coverage   47.24%   57.13%   +9.89%     
==========================================
  Files         476      549      +73     
  Lines       46892    60304   +13412     
==========================================
+ Hits        22154    34456   +12302     
- Misses      24738    25848    +1110     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.