Fix dangling deleted sessions after shutdown failures

[lionel-]

Fixes #15781

Release Notes

New Features

• N/A

Bug Fixes

• N/A

Validation Steps

@:sessions @:console @:ark

The positron-r and positron-supervisor extension-host suites cover the regression directly.

Manual checks:

1. Restart after a crash: kill the R process from a terminal, click Restart in the console, run 1 + 1, check completions, then delete the console. Each step should work, and the delete should not hang.
2. Normal delete: delete an idle R or Python console. It closes as before, with no force quit (nothing about forcing appears in the Window log).

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:sessions @:console @:ark @:interpreter

Why these tags?

Tag	Source
@:critical	Always runs (required)
@:sessions	PR description
@:console	PR description
@:ark	PR description
@:interpreter	Changed files

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