Annotate TryLookupByType for nullability

[martincostello]

Supersedes #3662 to resolve #3659.

[Copilot]

Pull request overview

This PR adds nullability annotations to the TryLookupByType method to improve null-safety analysis by indicating that the out parameter is guaranteed to be non-null when the method returns true. The implementation has been refactored to support this annotation pattern.

• Added [NotNullWhen(true)] attribute to the referenceSchema out parameter
• Refactored TryLookupByType implementation to explicitly initialize the out parameter before conditional assignment
• Removed private set from Schemas property accessor

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.67%. Comparing base (e28f031) to head (ac97564).
⚠️ Report is 63 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3719      +/-   ##
==========================================
+ Coverage   94.53%   94.67%   +0.13%     
==========================================
  Files         111      111              
  Lines        3861     3868       +7     
  Branches      778      780       +2     
==========================================
+ Hits         3650     3662      +12     
+ Misses        211      206       -5     

Flag	Coverage Δ
Linux	94.67% <100.00%> (+0.13%)	⬆️
Windows	94.67% <100.00%> (+0.13%)	⬆️
macOS	94.67% <100.00%> (+0.13%)	⬆️

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