Missing or inconsistent syntax highlighting across UI components

[KJ7LNW]

Problem

Several UI components display content without proper syntax highlighting:

• File listings (listFilesTopLevel, listFilesRecursive, searchFiles) use generic 'log' highlighting
• Code definitions (listCodeDefinitionNames) display as plain text
• Instruction content (fetchInstructions) lacks markdown formatting
• Newly created files don't use language detection based on file extension
• Some diff displays are inconsistently highlighted
• Error messages use plaintext instead of proper XML highlighting
• The language property is optional in CodeAccordian, allowing undefined values

Solution

Add appropriate syntax highlighting for each component:

• Change file listings to use 'shellsession' for terminal-like highlighting
• Use 'markdown' for code definitions and instructions
• Add file extension-based language detection for new files
• Ensure consistent 'diff' highlighting for all diff content
• Use 'xml' language for error messages
• Make language property required in CodeAccordian
• Set default fallback to 'txt' instead of undefined

These changes will improve readability and make it easier to distinguish between different types of content throughout the UI.