Clarify test WorkItem requirements in agent instructions

[Copilot]

This updates the repo’s agent guidance so test-related instructions consistently require WorkItem to appear directly alongside the test attribute, using the GitHub issue URL form. The goal is to make the expected test annotation pattern explicit and uniform across authoring and review guidance.

• Scope

  • Updated the shared Copilot instructions and the compiler, IDE, and Razor instruction files.
  • Updated skill guidance used for analyzer/code-fix work and code review.

• Behavioral guidance

  • Replaced looser phrasing like “add WorkItem for issue-based tests” with a single explicit rule:
    • always pair the test attribute with WorkItem
    • use the GitHub issue URL form
    • replace the sample issue number with the actual issue being addressed

• Canonical example

  [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/1234")]

• Why this matters

  • Removes ambiguity about placement and format.
  • Keeps test authoring guidance aligned across instruction sources and review surfaces.

Microsoft Reviewers: Open in CodeFlow

[JoeRobich]

Do we need this in each area's instruction if it is already in the base instructions?

[akhera99]

Out of curiosity, where is this not working currently? The base instruction seems to get picked up by the CLI -- I've never had to prompt to add the workitem attributes there. Is this not working on VS/VSCode/GitHub?

[davidwengier]

The "always" wording here is interesting. Does it now add workitem attributes even if tests don't come from an obvious bug or issue?