Preserve nullable struct null-check semantics in UA0003 (#4039)

## 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 #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)

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/Opc.Ua.MigrationAnalyzer.Tests/Analyzers/UA0003Tests.cs

@@ -123,6 +123,42 @@ namespace Opc.Ua.MigrationAnalyzer.Tests.Analyzers
             Assert.That(diags.Any(d => d.Id == "UA0003"), Is.False);
         }
 
+        [Test]
+        public async Task DoesNotReportOnNullableNodeIdEqualsNullAsync()
+        {
+            const string source = """
+                using Opc.Ua;
+                class C
+                {
+                    static bool M(NodeId? n) => n == null;
+                }
+                """;
+
+            ImmutableArray<Diagnostic> diags = await AnalyzerHarness
+                .GetAnalyzerDiagnosticsAsync(new UA0003NullCheckOnStructTypeAnalyzer(), source)
+                .ConfigureAwait(false);
+
+            Assert.That(diags.Any(d => d.Id == "UA0003"), Is.False);
+        }
+
+        [Test]
+        public async Task DoesNotReportOnNullableLocalizedTextNotEqualsNullAsync()
+        {
+            const string source = """
+                using Opc.Ua;
+                class C
+                {
+                    static bool M(LocalizedText? value) => null != value;
+                }
+                """;
+
+            ImmutableArray<Diagnostic> diags = await AnalyzerHarness
+                .GetAnalyzerDiagnosticsAsync(new UA0003NullCheckOnStructTypeAnalyzer(), source)
+                .ConfigureAwait(false);
+
+            Assert.That(diags.Any(d => d.Id == "UA0003"), Is.False);
+        }
+
         [Test]
         public async Task DoesNotReportOnEqualsMethodCallAsync()
         {