sessions: add dynamic codicon chat backgrounds

[TylerLeonhardt]

Summary

• add a responsive, theme-aware Codicons chat background preset generated from the existing in-memory Codicon font
• reconcile Set Background..., Clear Background, and Change Background Layout... across preset and image backgrounds
• preview image layouts while navigating the picker, committing on completion and restoring the configured layout on cancellation or save failure
• keep up to five machine-local recently used images for quick reuse
• preserve accessibility by keeping the decorative layer inert and suppressing backgrounds in high-contrast themes

Testing

• npm run compile
• npm run typecheck-client
• .\scripts\test.bat src\vs\sessions\contrib\chat\test\browser\chatView.test.ts src\vs\sessions\services\chatBackground\test\browser\chatBackgroundService.test.ts (25 passing)
• node --experimental-strip-types build\hygiene.ts <all 9 changed files>
• git diff --check origin/main...HEAD
• manually exercised new-chat and active-chat views in Code OSS across light, dark, and high-contrast themes; verified command visibility, responsive Codicon regeneration, layout preview/rollback, and recent-image persistence

Reviewer note

The full npm run hygiene wrapper currently fails in this Windows checkout before source checks because it resolves paths with a duplicated C:\C:\... prefix. Running build\hygiene.ts directly against every changed file passes.

[Copilot]

Pull request overview

Adds configurable, theme-aware Codicon and recent-image backgrounds to Sessions chat.

Changes:

• Adds responsive decorative Codicon rendering.
• Adds background presets, recent images, and layout previews.
• Updates commands, accessibility help, styling, and tests.

Show a summary per file

File	Description
contextkeys.ts	Adds configured-background context.
chatBackgroundService.ts	Manages presets, layouts, and recents.
chatBackgroundService.test.ts	Tests new service behavior.
chatBackgroundRenderer.ts	Renders image and Codicon backgrounds.
chatView.ts	Integrates the renderer.
chatView.css	Styles background layers and content.
chat.contribution.ts	Updates commands and settings.
sessionsChatAccessibilityHelp.ts	Documents accessible background controls.
chatView.test.ts	Tests renderer behavior and styling.

Review details

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• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Balanced

[Justin Chen (justschen)]

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