[Geneva] Add additional Encoding overrides

[martincostello]

Changes

Add missing overrides that skip byte counting.

I spotted this while make changes for C# 15 in #3867 (db267c2).

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 3.03030% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.48%. Comparing base (8293304) to head (54e21fd).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rter.Geneva/Internal/Tld/UncheckedASCIIEncoding.cs	3.03%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4972      +/-   ##
==========================================
- Coverage   77.52%   77.48%   -0.04%     
==========================================
  Files         468      468              
  Lines       19920    19947      +27     
==========================================
+ Hits        15442    15456      +14     
- Misses       4478     4491      +13     

Flag	Coverage Δ
unittests-Exporter.Geneva	56.44% <3.03%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rter.Geneva/Internal/Tld/UncheckedASCIIEncoding.cs	11.39% <3.03%> (-5.92%)	⬇️

... and 2 files with indirect coverage changes

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[martincostello]

Given the coverage stats, it looks like nothing currently uses any code paths that make these actually make a difference. We could re-open in future if that changes.