Add Run and Debug session state regression tests

[Sean McManus (sean-mcmanus)]

Summary

Add regression coverage for Run and Debug session state when a C++ source file is outside the open workspace and when the active document changes language. This follows up on the coverage deferred from #14719.

This PR was investigated and created by GitHub Copilot in VS Code. Any message starting with ✨Copilot: was sent by Copilot.

Validation

• yarn compile
• DISPLAY=:0 yarn ts-node -T .scripts/test.ts --scenario=SingleRootProject (56 passing)
• Removed the production onDidOpenTextDocument listener as a reversible mutation; the new language-transition test failed because buildAndDebugIsSourceFile remained false after switching back to C++, then passed after restoration.

Context

The follow-up was requested in the review discussion on #14719.

[Copilot]

🟢 Approval recommended

The tests directly cover the previously deferred external-file and language-transition regressions.

Pull request overview

Adds regression coverage for session-state tracking introduced in #14719.

Changes:

• Tests external C++ files outside the workspace.
• Tests session-state updates across language-mode changes.
• Adds asynchronous state assertions and temporary-file cleanup.

File summaries

File	Description
Extension/test/scenarios/SingleRootProject/tests/buildAndDebug.test.ts	Adds focused Run and Debug session-state regression tests.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Sean McManus (sean-mcmanus)]

✨Copilot: Prashant Kumar Rai (@8prashant) This is the follow-up regression coverage from #14719. GitHub does not allow requesting reviews from non-collaborators, but your feedback here is welcome.

[Prashant Kumar Rai (8prashant)]

Hi Sean McManus (@sean-mcmanus), I reviewed the follow-up PR. The new tests cover the two scenarios from PR #14719:

• An external C++ source file is correctly identified as a source file but not as part of the workspace.
• The session state updates correctly when the active document changes from C++ to plaintext and back to C++.

The tests also exercise the actual language-change behavior and include cleanup for the temporary files. The compile and scenario test results are passing, and I did not find any blocking issues.

Approved from my side.