TUNU0001 code fixer crashes on multi-targeting projects

[thomhurst]

Description

The TUNU0001 NUnit migration code fixer crashes when running dotnet format analyzers --severity info --diagnostics TUNU0001 on projects that target multiple frameworks.

Reproduction Steps

1. Create a test project with multiple target frameworks:

<PropertyGroup>
    <TargetFrameworks>net8.0;net9.0;net10.0;net462</TargetFrameworks>
</PropertyGroup>

2. Add TUnit and NUnit packages:

<PackageReference Include="TUnit" Version="1.11.*" />
<PackageReference Include="NUnit" Version="3.14.0" />

3. Run the code fixer:

dotnet format analyzers --severity info --diagnostics TUNU0001

Expected Behavior

The code fixer should process all files and apply migrations.

Actual Behavior

The code fixer crashes with the following exception:

Unhandled exception: System.AggregateException: One or more errors occurred. (Changes must be within bounds of SourceText (Parameter 'changes'))
 ---> System.ArgumentException: Changes must be within bounds of SourceText (Parameter 'changes')
   at Microsoft.CodeAnalysis.Text.SourceText.WithChanges(IEnumerable`1 changes)
   at Microsoft.CodeAnalysis.LinkedFileMergeConflictCommentAdditionService.GetCommentChangesForDocument(...)
   ...

Workaround

Temporarily change the project to use a single target framework:

<TargetFramework>net10.0</TargetFramework>

Then run the code fixer, and restore multi-targeting afterward.

Environment

• TUnit version: 1.11.64
• .NET SDK: 10.0.102
• OS: Windows

Suggested Fix

The code fixer should handle multi-targeting projects by processing files for a single TFM at a time, or by properly merging changes across linked file instances.

Tests should be added to verify the code fixer works correctly with multi-targeting projects.

[claude]

This appears to be a duplicate of #4484. Both issues describe the same underlying problem - code fixers crash on multi-targeting projects with the error "Changes must be within bounds of SourceText".

While this issue is for TUNU0001 (NUnit migration) and #4484 is for TUXU0001 (xUnit migration), the root cause is identical - both code fixers fail to properly handle linked files in multi-targeting projects.

The fix should address both analyzers since they likely share the same code fixer infrastructure.