Fix focus stealing when clearing terminal scrollback

[w3lld1]

Summary

• I map powershell.integratedConsole.forceClearScrollbackBuffer into the language-server settings.
• I emit xterm's CSI 3 J sequence directly from PSES when the setting is enabled, so clearing saved lines no longer invokes the focus-stealing client command.
• I added regression coverage for settings updates and the in-band terminal output.

Validation

• Invoke-Build -Configuration Release TestFull (351 unit tests passed with 1 platform skip; 48 stable-PowerShell E2E tests passed with 2 platform skips; 48 preview-PowerShell E2E tests passed with 2 platform skips)
• Focused Release EditorOperationsServiceTests (4 passed)
• dotnet format PowerShellEditorServices.sln --no-restore --verify-no-changes --include src/PowerShellEditorServices/Services/Workspace/LanguageServerSettings.cs src/PowerShellEditorServices/Services/Extension/EditorOperationsService.cs test/PowerShellEditorServices.Test/Extensions/EditorOperationsServiceTests.cs
• git diff --check upstream/main...HEAD

The Windows PowerShell 5.1-only legs were not available on this Linux host and remain covered by upstream CI.

Fixes #2313

[Copilot]

Pull request overview

Maps the terminal scrollback setting into PSES and clears saved lines in-band, avoiding the focus-stealing client command.

Changes:

• Adds integrated-console settings handling.
• Emits xterm’s CSI 3 J sequence when configured.
• Adds regression tests for configuration updates and terminal output.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
LanguageServerSettings.cs	Maps the scrollback setting into PSES configuration.
EditorOperationsService.cs	Clears scrollback directly without a client notification.
EditorOperationsServiceTests.cs	Tests settings propagation and emitted output.

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