Support light/dark theme for agent icons

[rebornix]

Re #273345

[Copilot]

Pull Request Overview

This PR adds support for light and dark theme variants for chat session agent icons. Extensions can now specify different icon paths for light and dark themes, similar to how VS Code handles theme-specific icons elsewhere in the product. The implementation extends the chat sessions extension point to accept either a string (for codicon IDs) or an object with light and dark properties (for file paths), and updates the icon resolution logic to return the appropriate icon based on the current theme.

Key Changes

• Extended the IChatSessionsExtensionPoint interface to support icon objects with light and dark properties
• Updated icon resolution to use IThemeService to determine which theme-specific icon to return
• Improved icon parsing to distinguish between codicon IDs and file paths

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/vs/workbench/contrib/chat/common/chatSessionsService.ts	Updated icon type to support theme-specific icon objects
src/vs/workbench/contrib/chat/browser/chatSessions.contribution.ts	Added schema for light/dark icon objects, updated icon parsing logic, and theme-aware icon resolution
src/vs/workbench/contrib/chat/browser/chatEditorInput.ts	Removed redundant caching code in getIcon() method
src/vs/workbench/contrib/chat/test/common/mockChatSessionsService.ts	Added type check to handle both string and object icon types

resolved