Migrate to .slnx solution file using dotnet sln migrate

[Copilot]

This PR migrates the repository to use the new .slnx (XML-based solution) file format for improved compatibility with modern .NET tooling and Visual Studio.

Changes

• Generated EssentialCSharp.ListingManager.slnx using the dotnet sln migrate command
• Removed the legacy EssentialCSharp.ListingManager.sln file to avoid CI/CD ambiguity errors when multiple solution files are present

Benefits

The .slnx format is a new XML-based solution file format introduced in .NET 9 that provides:

• Cleaner, more readable XML structure
• Better diff-ability in version control
• Forward compatibility with future .NET tooling
• Interoperability with Visual Studio 2022 and later versions
• Resolves CI/CD workflow errors caused by having multiple solution files

Verification

The .slnx file has been tested and works correctly:

# Restore, build, and test all work with the .slnx file
dotnet restore
dotnet build
dotnet test

# List projects in the solution
dotnet sln EssentialCSharp.ListingManager.slnx list

The .slnx file contains the same two projects and solution items as the original .sln file:

• ListingManager/EssentialCSharp.ListingManager.csproj
• ListingManager.Tests/EssentialCSharp.ListingManager.Tests.csproj
• Solution Items (Directory.Build.props, Directory.Packages.props, NuGet.config, README.md)

Resolves the requirement to migrate to the new solution format.

Original prompt

run dotnet sln migrate (https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-sln#migrate) to add in a .slnx file to live alongside our .sln file for now (.sln for backwards compatibility for now)

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[BenjaminMichaelis]

@copilot maybe we can drop the .sln file and just keep the .slnx?

We get this otherwise.

Run dotnet restore
dotnet restore
shell: /usr/bin/pwsh -command ". '{0}'"
env:
DOTNET_ROOT: /usr/share/dotnet
MSBUILD : error MSB1011: Specify which project or solution file to use because this folder contains more than one project or solution file.
Error: Process completed with exit code 1.

[Copilot]

@copilot maybe we can drop the .sln file and just keep the .slnx?

We get this otherwise.
...

Done! Removed the .sln file in commit 77884b5. The repository now uses only the .slnx file, which resolves the ambiguity error in CI/CD workflows. Verified that dotnet restore, dotnet build, and dotnet test all work correctly with just the .slnx file.