Add Cesium Copilot AI assistant to Sandcastle

[jdehorty]

Summary

This PR introduces Cesium Copilot, an AI-powered coding assistant integrated directly into Sandcastle. Copilot helps users write, modify, and debug CesiumJS code through a conversational interface with intelligent code generation and real-time diff previews.

Features

Multi-Model AI Support

• Google Gemini (Gemini 3 Flash Preview, Gemini 3 Pro Preview)
• Anthropic Claude (Claude Opus 4.5, Claude Sonnet 4.5, Claude Haiku 4.5)
• Easy model switching via dropdown picker
• Secure API key management with localStorage persistence

Intelligent Code Assistance

• Contextual Understanding: Copilot sees your current JavaScript and HTML code
• CesiumJS-Aware Prompts: System prompts include API deprecation guides to avoid suggesting outdated patterns (e.g., createWorldTerrain() → Terrain.fromWorldTerrain())
• Streaming Responses: Real-time token streaming for responsive feedback
• Extended Thinking: Support for Claude's extended thinking mode with reasoning display

Code Editing with Diff Preview

• Search/Replace Diff Format: AI generates precise code edits using a structured diff format
• Visual Diff Preview: Side-by-side comparison before applying changes
• Fuzzy Matching: Robust diff application even when code has minor variations
• One-Click Apply: Accept or reject proposed changes with full undo support

Tool Use Integration

• Run Code Tool: Execute code in Sandcastle and capture results/errors
• Error Context Extraction: Automatic error detection with line numbers and stack traces
• Auto-Iteration: Optional automatic retry on errors with oscillation detection

Conversation Management

• Chat History: Persistent conversation history with search
• Multi-Turn Context: Full conversation context maintained across messages
• Image Attachments: Paste or drag-and-drop images for visual context

User Experience

• Collapsible chat panel with keyboard shortcuts
• Settings panel for model configuration and feature toggles
• Welcome guidance for new users
• Token usage display

Architecture

packages/sandcastle/src/
├── AI/
│   ├── AIClientFactory.ts      # Factory for creating AI clients
│   ├── AnthropicClient.ts      # Claude API integration
│   ├── GeminiClient.ts         # Gemini API integration  
│   ├── ApiKeyManager.ts        # Secure key storage
│   ├── DiffParser.ts           # Parse AI-generated diffs
│   ├── DiffMatcher.ts          # Fuzzy matching for diff application
│   ├── DiffApplier.ts          # Apply diffs to source code
│   ├── EditParser.ts           # Parse edit instructions
│   ├── ErrorContext.ts         # Extract error information
│   ├── prompts.ts              # System prompts with CesiumJS knowledge
│   ├── tools/toolRegistry.ts   # Tool definitions for function calling
│   └── types.ts                # TypeScript interfaces
├── ChatPanel.tsx               # Main chat interface
├── ChatMessage.tsx             # Message rendering with markdown
├── DiffPreview.tsx             # Visual diff component
├── components/
│   ├── settings/               # Settings UI components
│   ├── history/                # Conversation history sidebar
│   └── controls/               # Model picker, toggles
└── contexts/                   # React contexts for state management

Testing

• 16 test files covering core functionality
• Unit tests for diff parsing, matching, and application
• Component tests for UI interactions
• Integration tests for AI client behavior

Screenshots

[Screenshots would be added here showing the chat panel, diff preview, and settings]

Checklist

• Code compiles without errors
• Tests pass locally
• No changes to core CesiumJS engine
• Changes isolated to packages/sandcastle
• API keys are never logged or exposed
• Added contributor entry

Notes

• This feature is opt-in: users must provide their own API keys
• No API keys are stored on any server; all keys remain in browser localStorage
• The AI has read-only context of the current code; all edits require user approval

[github-actions]

Thank you for the pull request, @jdehorty!

✅ We can confirm we have a CLA on file for you.