fix(sdk): fallback to get_project screenInstances when list_screens returns empty (#149)

[davideast]

Summary

Resolves #149 (Tier 2 — Part 1 of 3 in stack).

When a new project has screens generated via project.generate(), the Stitch backend populates get_project's screenInstances array (sourceScreen: "projects/{projectId}/screens/{screenId}") and returns the generated Screen objects in generate_screen_from_text, but the MCP list_screens endpoint returns [] until the project is manually opened in stitch.withgoogle.com.

Changes

• EntityManager.prototype.getCachedScreensData(projectId): Returns raw screen payloads for any Screen instances already resolved in the current session for projectId.
• StitchToolClient.prototype.callTool("list_screens", { projectId }):
  • When list_screens returns [], automatically falls back to get_project (screenInstances) and hydrates each sourceScreen via get_screen (falling back to ScreenInstance metadata if get_screen is unavailable), combined with any in-session cached screens in EntityManager.
  • When list_screens returns a subset of screens (stale index after an in-session project.generate()), merges any missing in-session generated screens from EntityManager.
• Fixes both await project.screens() and await project.downloadAssets() immediately after project.generate().