Add IReactiveObject source generator and tests

[ChrisPulman]

What kind of change does this PR introduce?

feature

What is the new behavior?

Introduces the [IReactiveObject] attribute and a new source generator to implement IReactiveObject for classes that cannot inherit from ReactiveObject.
Updates documentation, adds generator logic, supporting models, and unit tests to verify generated code.
Also updates helper and project files to support the new generator.

What might this PR break?

None, new feature

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[Copilot]

Pull request overview

This PR introduces a new source generator that implements the IReactiveObject interface for classes that cannot inherit from ReactiveObject. The generator is triggered by the [IReactiveObject] attribute and generates the necessary event infrastructure and interface implementation.

Key changes:

• Added ReactiveObjectGenerator with attribute-based code generation for IReactiveObject interface implementation
• Updated helper classes to support the new generator and consolidated common constants
• Added unit tests with verified snapshots for the new generator

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/ReactiveUI.SourceGenerators.Roslyn/ReactiveObject/ReactiveObjectGenerator.cs	Main generator class that registers the IReactiveObject attribute and orchestrates code generation
src/ReactiveUI.SourceGenerators.Roslyn/ReactiveObject/ReactiveObjectGenerator.Execute.cs	Contains execution logic for analyzing classes and generating IReactiveObject implementation code
src/ReactiveUI.SourceGenerators.Roslyn/ReactiveObject/Models/ReactiveObjectInfo.cs	Data model for storing information about classes to be processed by the generator
src/ReactiveUI.SourceGenerators.Roslyn/IViewFor/IViewForGenerator.Execute.cs	Updated to use shared ExcludeFromCodeCoverage constant from AttributeDefinitions
src/ReactiveUI.SourceGenerators.Roslyn/Core/Extensions/FieldSyntaxExtensions.cs	Updated to use centralized ReactiveObjectAttributeType constant for consistency
src/ReactiveUI.SourceGenerators.Roslyn/BindableDerivedList/BindableDerivedListGenerator.cs	Removed unused using directives
src/ReactiveUI.SourceGenerators.Roslyn/AttributeDefinitions.cs	Added IReactiveObjectAttribute definition and ExcludeFromCodeCoverage constant
src/ReactiveUI.SourceGenerators.Execute/Person.cs	Updated example to use the new IReactiveObject attribute instead of inheriting from ReactiveObject
src/ReactiveUI.SourceGenerator.Tests/UnitTests/ReactiveObjectGeneratorTests.cs	Added unit test for the ReactiveObject generator
src/ReactiveUI.SourceGenerator.Tests/TestHelper.cs	Added REACTIVEOBJ folder mapping for test output organization
src/ReactiveUI.SourceGenerator.Tests/ReactiveUI.SourceGenerators.Tests.csproj	Added REACTIVEOBJ test folder to project structure
src/ReactiveUI.SourceGenerator.Tests/REACTIVEOBJ/...	Added verified test snapshots for generated code
README.md	Added documentation for the new IReactiveObject attribute with usage example

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[codecov]

Codecov Report

❌ Patch coverage is 94.26230% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.86%. Comparing base (8aedecb) to head (826db8d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../ReactiveObject/ReactiveObjectGenerator.Execute.cs	92.59%	4 Missing and 2 partials ⚠️
...ators.Roslyn/IViewFor/IViewForGenerator.Execute.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
+ Coverage   40.12%   42.86%   +2.74%     
==========================================
  Files          61       64       +3     
  Lines        3469     3583     +114     
  Branches      399      404       +5     
==========================================
+ Hits         1392     1536     +144     
+ Misses       1967     1935      -32     
- Partials      110      112       +2     

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