feat: persist chat input text across VSCode restarts

[hannesrudolph]

Summary

Persist the chat input text across VSCode restarts and crashes at the workspace level, so each workspace maintains its own draft message. This ensures users don't lose their work-in-progress messages if the extension crashes or VSCode restarts.

Changes

Backend (src/)

• Added DRAFT_MESSAGE_KEY constant and draft management methods to ClineProvider.ts:
  • saveDraftMessage(text, images) - saves draft to workspace state
  • getDraftMessage() - retrieves saved draft
  • clearDraftMessage() - clears the draft
• Added webview message handlers for saveDraftMessage, getDraftMessage, and clearDraftMessage

Types

• Added saveDraftMessage, getDraftMessage, clearDraftMessage to WebviewMessage types
• Added draftMessage to ExtensionMessage types

Frontend (webview-ui/)

• Added debounced draft save (500ms) to prevent excessive writes
• Request saved draft on component mount
• Handle draftMessage response to restore draft
• Created handleInputValueChange wrapper to auto-save draft
• Clear draft on message send and chat reset

Tests

• Added unit tests for draft message webview handlers

Behavior

• Draft messages are saved at the workspace level (each workspace has its own draft)
• Both text and images are persisted
• Drafts are cleared when:
  • A message is sent
  • The user starts a new task
• Drafts persist across:
  • Webview reloads
  • Extension restarts
  • VSCode restarts
  • VSCode crashes (workspaceState is disk-backed)
  • Task switching (viewing different tasks in history)

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