fix: close singleton editors when using 'Copy into New Window'

[Rani367]

Summary

Fixes singleton editors (webviews, terminals, etc.) being shared between
windows when using "Copy into New Window".

Problem

When using "Copy into New Window" on a webview editor (like the integrated browser):

• Both windows shared the same webview instance
• Interactions were mirrored between windows
• Performance lag/glitching occurred in animations

Solution

Treat singleton editors as moves even when the action is "copy" -
the original editor is closed after opening in the new window.

Test Plan

• Added unit tests for singleton editor copy behavior
• Added unit test for onWillMoveEditor event firing
• All 64 editor group tests pass

Fixes #284525

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/browser/parts/editor/editorGroupView.ts
• src/vs/workbench/services/editor/test/browser/editorGroupsService.test.ts

[Rani367]

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