[WIP] Create source-only filter package

[Copilot]

Thanks for the feedback on #15638. I've created this new PR, which merges into #15638, to address your comment. I will work on the changes and keep this PR's description up to date as I make progress.

Original PR: #15638
Triggering comment (#15638 (comment)):

@copilot Create a new test project for the new package.

You MUST:

• Open a separate PR for that. Do not push to this PR.
• The new test project shouldn't reference the csproj. Instead, it should manually add the C# source files to Compile item.
• Write basic tests to validate the behavior of the APIs.