feat: add SpriteAtlas tool definitions and bridge functions

7 new tools for SpriteAtlas management:
- unity_spriteatlas_create
- unity_spriteatlas_info
- unity_spriteatlas_add
- unity_spriteatlas_remove
- unity_spriteatlas_settings
- unity_spriteatlas_delete
- unity_spriteatlas_list

Requires companion plugin PR with MCPSpriteAtlasCommands.cs
and route registration in MCPBridgeServer.cs.