build: treat warnings as errors; fix CS0419 cref ambiguity

Enable TreatWarningsAsErrors across the build. The code-fix project
vendored its own copies of the shared helpers/extensions in the same
namespaces as the generator (ReactiveUI.SourceGenerators.Helpers /
.Extensions), so the test project — which references both assemblies —
saw two metadata copies of each type and the doc-comment cref binder
(which ignores accessibility) reported CS0419.

- Move the code-fix project's vendored helpers/extensions under
  ReactiveUI.SourceGenerators.CodeFixers.Helpers / .Extensions so the
  type identities no longer collide with the generator's.
- Fully-qualify the SymbolExtensionTests crefs that collided with the
  Microsoft.CodeAnalysis.ISymbolExtensions/ITypeSymbolExtensions types.

Whole solution builds warning-clean with -warnaserror; 891 tests pass on
net8/9/10.

Author: glennawatson

src/Directory.Build.props

@@ -28,7 +28,7 @@
     <LangVersion>preview</LangVersion>
     <EnableNETAnalyzers>True</EnableNETAnalyzers>
     <AnalysisLevel>latest</AnalysisLevel>
-    <WarningsAsErrors>nullable</WarningsAsErrors>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <IncludePackageReferencesDuringMarkupCompilation>true</IncludePackageReferencesDuringMarkupCompilation>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>

src/ReactiveUI.SourceGenerator.Tests/UnitTests/SymbolExtensionTests.cs

@@ -8,8 +8,10 @@
 namespace ReactiveUI.SourceGenerator.Tests;
 
 /// <summary>
-/// Unit tests for <see cref="ISymbolExtensions"/>, <see cref="ITypeSymbolExtensions"/>,
-/// <see cref="INamedTypeSymbolExtensions"/>, and <see cref="CompilationExtensions"/>.
+/// Unit tests for <see cref="ReactiveUI.SourceGenerators.Extensions.ISymbolExtensions"/>,
+/// <see cref="ReactiveUI.SourceGenerators.Extensions.ITypeSymbolExtensions"/>,
+/// <see cref="ReactiveUI.SourceGenerators.Extensions.INamedTypeSymbolExtensions"/>, and
+/// <see cref="ReactiveUI.SourceGenerators.Extensions.CompilationExtensions"/>.
 /// All tests compile small in-memory programs to obtain real Roslyn symbol instances.
 /// </summary>
 public sealed class SymbolExtensionTests

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Core/Extensions/FieldSyntaxExtensions.cs

@@ -5,7 +5,7 @@
 
 using Microsoft.CodeAnalysis;
 
-namespace ReactiveUI.SourceGenerators.Extensions;
+namespace ReactiveUI.SourceGenerators.CodeFixers.Extensions;
 
 internal static class FieldSyntaxExtensions
 {

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Core/Extensions/ISymbolExtensions.cs

@@ -5,7 +5,7 @@
 
 using Microsoft.CodeAnalysis;
 
-namespace ReactiveUI.SourceGenerators.Extensions;
+namespace ReactiveUI.SourceGenerators.CodeFixers.Extensions;
 
 /// <summary>
 /// Extension methods for the <see cref="ISymbol"/> type.

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Core/Extensions/ITypeSymbolExtensions.cs

@@ -5,9 +5,9 @@
 
 using System;
 using Microsoft.CodeAnalysis;
-using ReactiveUI.SourceGenerators.Helpers;
+using ReactiveUI.SourceGenerators.CodeFixers.Helpers;
 
-namespace ReactiveUI.SourceGenerators.Extensions;
+namespace ReactiveUI.SourceGenerators.CodeFixers.Extensions;
 
 /// <summary>
 /// Extension methods for the <see cref="ITypeSymbol"/> type.

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Core/Helpers/EquatableArray{T}.cs

@@ -11,7 +11,7 @@
 using System.Linq;
 using System.Runtime.CompilerServices;
 
-namespace ReactiveUI.SourceGenerators.Helpers;
+namespace ReactiveUI.SourceGenerators.CodeFixers.Helpers;
 
 /// <summary>
 /// An immutable, equatable array. This is equivalent to <see cref="ImmutableArray{T}"/> but with value equality support.

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Core/Helpers/ImmutableArrayBuilder{T}.cs

@@ -11,7 +11,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
-namespace ReactiveUI.SourceGenerators.Helpers;
+namespace ReactiveUI.SourceGenerators.CodeFixers.Helpers;
 
 /// <summary>
 /// A helper type to build sequences of values with pooled buffers.

src/ReactiveUI.SourceGenerators.Analyzers.CodeFixes/Diagnostics/CodeFixers/PropertyToReactiveFieldAnalyzer.cs

@@ -10,7 +10,7 @@
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Diagnostics;
-using ReactiveUI.SourceGenerators.Extensions;
+using ReactiveUI.SourceGenerators.CodeFixers.Extensions;
 using static ReactiveUI.SourceGenerators.CodeFixers.Diagnostics.DiagnosticDescriptors;
 
 namespace ReactiveUI.SourceGenerators.CodeFixers;