[FEATURE] VSCode extension code blocks should respect system/editor theme

Description

The VSCode extension renders code blocks in responses with what appears to be a dark syntax highlighting theme, regardless of the user's system theme or VSCode color theme settings.

Expected Behavior

Code blocks in Claude's responses should respect the user's VSCode color theme (or system theme when auto-detect is enabled), matching the syntax highlighting used in the editor itself.

Current Behavior

Code blocks appear to default to a dark theme syntax highlighting even when:

• System theme is set to light mode
• VSCode is configured with window.autoDetectColorScheme: true
• VSCode light theme is active (e.g., Solarized Light)

Environment

• Platform: macOS
• VSCode theme: Solarized Light/Dark with auto-detect enabled
• Extension: Claude Code VSCode extension

Additional Context

VSCode's built-in markdown preview and other extensions typically respect the active color theme for syntax highlighting. This would improve accessibility and reduce eye strain for users who switch between light/dark modes.

Related terminal issues: #2990, #1148 (but those are for TUI, not VSCode extension)