sessions: make changes diffs responsive

[Sandeep Somavarapu (sandy081)]

Summary

• make both the Agents Window Session Changes editor and standard text diff editors prefer side-by-side while automatically falling back to inline when space is limited
• share and persist one Agents Window diff-layout preference across both supported surfaces without changing normal workbench diff behavior or workspace configuration
• clarify the preference action labels, tooltips, checked state, and accessibility help
• keep every visible Agents Window text diff synchronized when the preference changes

Validation

• npm run compile
• npm run hygiene
• npm run valid-layers-check
• targeted unit tests (17 passing)

[Copilot]

Pull request overview

Makes Agents Window diffs responsive while preserving a Sessions-specific layout preference.

Changes:

• Adds persisted side-by-side preference for Session Changes.
• Enables responsive inline fallback in multi-diff and single-file editors.
• Updates actions, accessibility help, and tests.

Show a summary per file

File	Description
src/vs/sessions/contrib/editor/test/browser/diffEditor.sessions.contribution.test.ts	Tests Sessions-specific toggling.
src/vs/sessions/contrib/editor/browser/diffEditor.sessions.contribution.ts	Applies configured responsive layouts.
src/vs/sessions/contrib/changes/test/browser/changesViewActions.test.ts	Updates action metadata tests.
src/vs/sessions/contrib/changes/browser/sessionsChangesAccessibilityHelp.ts	Describes responsive preferences.
src/vs/sessions/contrib/changes/browser/sessionChangesEditor.ts	Persists and applies the preferred layout.
src/vs/sessions/contrib/changes/browser/changesViewActions.ts	Revises toggle labels, state, and tooltips.
src/vs/editor/test/browser/widget/multiDiffEditorWidget.test.ts	Tests responsive multi-diff behavior.
src/vs/editor/browser/widget/multiDiffEditor/multiDiffEditorWidgetImpl.ts	Propagates diff layout options.
src/vs/editor/browser/widget/multiDiffEditor/multiDiffEditorWidget.ts	Extends the layout override API.

Review details

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• Files reviewed: 9/9 changed files
• Comments generated: 4
• Review effort level: Balanced