Description

Part 2 of 2. Builds the Cesium Copilot UI on top of the engine from #13413 and wires it into the Sandcastle application.

• Chat UI: ChatPanel, ChatMessage (markdown rendering via react-markdown + remark-gfm), ThinkingBlock, ThinkingAccordion, ToolCallDisplay, PromptInput.
• Diff review UI: DiffPreview, SimpleDiffPreview, StreamingDiffPreview, DiffReviewPanel, InlineChangeWidget.
• Settings and model selection: ApiKeyDialog, ModelPicker, AdvancedSettings, FeatureSettings, SettingsPanel, SettingsContext, SettingsProvider.
• State: ModelContext, useModel, plus hooks (useAutoFix, useChatMessages, useToolChainExecution).
• App integration: App.tsx mounts the Copilot pane, SandcastleEditor exposes code context, Bucket / ConsoleMirror / util adjustments for console streaming and iframe bridging, plus ErrorBoundary, icon additions, and a chat logo.

New runtime dependencies: react-markdown, remark-gfm, react-virtuoso.

Base branch

This PR targets cesium-copilot-engine (not main), so GitHub shows only the UI/integration diff. Once part 1 merges to main, this PR auto-retargets to main.

Issue number and link

N/A (feature branch).

Testing plan

• tsc -b passes inside packages/sandcastle/ standalone (with part 1's changes present).
• Chat panel opens, messages render with markdown, thinking blocks expand, tool calls display.
• Diff preview and review flow works end-to-end.
• API key entry and model switching persist across reload.
• Full validation via the cesium-copilot integration branch deploying to the sandbox.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md (added in part 1)
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

AI acknowledgment

• I used AI to generate content in this PR
• If yes, I have reviewed the AI-generated content before submitting