Preserve nullable struct null-check semantics in UA0003

[marcschier]

Failure

UA0003 unwrapped Nullable<T> before classifying OPC UA built-in structs. That
reported valid nullable == null and != null checks and offered fixes whose
.IsNull semantics differ from nullable-value absence.

Fix

• Leave System.Nullable<T> comparisons unchanged.
• Continue reporting direct null comparisons on OPC UA structs.
• Add coverage for nullable NodeId and LocalizedText operand orderings.

Reference

• OPC UA .NET 2.0 migration semantics.
• Split from Harden security and reliability across the OPC UA stack #4021.

Tests

• dotnet test tests\Opc.Ua.MigrationAnalyzer.Tests\Opc.Ua.MigrationAnalyzer.Tests.csproj -c Release -f net10.0 --nologo (121 passed)
• dotnet test tests\Opc.Ua.MigrationAnalyzer.Tests\Opc.Ua.MigrationAnalyzer.Tests.csproj -c Release -f net48 --nologo (121 passed)

[Copilot]

Pull request overview

Note

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Adjusts UA0003 to preserve C# nullable semantics by no longer unwrapping Nullable<T> before deciding whether a null-comparison should be reported/fixed.

Changes:

• Stop unwrapping Nullable<T> in UA0003 analysis, returning early to keep == null / != null checks intact.
• Add tests to ensure UA0003 does not report null-comparisons for nullable NodeId and LocalizedText (including operand ordering).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tools/Opc.Ua.MigrationAnalyzer/Analyzers/UA0003NullCheckOnStructTypeAnalyzer.cs	Changes analysis to skip nullable types, preserving Nullable<T> null-check behavior.
tests/Opc.Ua.MigrationAnalyzer.Tests/Analyzers/UA0003Tests.cs	Adds regression coverage ensuring nullable UA structs are not flagged, including null != value ordering.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.54%. Comparing base (70c74e7) to head (e15c475).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4039      +/-   ##
==========================================
- Coverage   73.55%   73.54%   -0.02%     
==========================================
  Files        1345     1345              
  Lines      180036   180037       +1     
  Branches    31677    31678       +1     
==========================================
- Hits       132427   132404      -23     
- Misses      36911    36922      +11     
- Partials    10698    10711      +13     

Files with missing lines	Coverage Δ
...r/Analyzers/UA0003NullCheckOnStructTypeAnalyzer.cs	90.47% <100.00%> (+4.42%)	⬆️

... and 21 files with indirect coverage changes

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[marcschier]

/azp run

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