Canvases: share definition refresh logic across Editor and Agents windows #341696

The Editor Window Canvas menu added by #341597 and the Agents Window Add Tab → Canvas menu added for microsoft/vscode-internalbacklog#10058 currently maintain separate canvas-definition refresh implementations. The broader Agents Window customization work originated in microsoft/vscode-internalbacklog#9997.

Both implementations independently handle:

• active owner/chat identity;
• asynchronous definition listing;
• preserving the current list during same-owner refreshes;
• structural equality for content-identical results;
• coalescing repeated invalidations;
• stale request and disposal guards;
• clearing definitions when the owner changes or canvases become unavailable.

The surface-specific adapters and menu placement should remain separate, but the common refresh lifecycle could live in a Workbench-owned helper so both vs/workbench and vs/sessions can consume it without reversing the layer dependency.

A possible shape is a CanvasDefinitionStore that accepts an owner key, invalidation signal, and asynchronous list callback, and exposes an observable definition list. Editor and Agents Window code would continue to own their context/session routing, menu IDs, ordering, and open actions.

Acceptance criteria

• Introduce a Workbench-layer canvas-definition refresh abstraction reusable by Editor and Agents windows.
• Preserve existing behavior for owner changes, unavailable canvases, failed refreshes, reconnects, and open native menus.
• Keep surface-specific menu contribution and canvas-opening behavior outside the shared helper.
• Retain focused regression coverage for stable commands, refresh coalescing, stale requests, and structurally identical results.
• Avoid visible UX changes.

Related implementation and fixes: #341495, #341498, #341597, and #341658.

🤖 Written by GitHub Copilot on behalf of Joaquín Ruales (@jruales).