Accept Box in element_builder schema

[hernang87]

Why

The Designer Extensions API now exposes elementPresets.Box, and the public MCP in the monolith accepts element_builder type: "Box" (webflow/webflow#122058). This server still rejects it: DEElementSchema fails with Invalid option before the request is forwarded to the Designer.

This PR adds Box to the DEElementSchema enum. It also updates the model-facing text to match the monolith:

• the type hint says to use Box for generic containers, that Section creates a Box with the section tag, and that Box elements come back as type Block;
• rules.ts lists Box among the elements that can have children.

Dependency: don't merge yet

This server only validates element_schema, then forwards it over RPC to the Designer bridge app, which maps element types to presets. Until that app maps Box to elementPresets.Box, a Box request will pass validation here and then fail in the Designer. I haven't confirmed where the bridge app's mapping lives; it may be the designer-app in mcp-remote-cloudflare-server, which needs the same enum change.

Testing

• npm run build and npx tsc --noEmit pass.
• DEElementSchema.safeParse({type: "Box"}) succeeds and an unknown type still fails.
• Not run: an end-to-end call against a Designer. Once the bridge app maps Box, element_builder with {type: "Box"} should create a Block with tag div.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CxAGfMvh2YLZpswVZtXLYE

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[gillevin-webflow]

LGTM as a schema change. Don't merge until Box is also mapped on the bridge side (mcp-remote-cloudflare-server: DEElementSchema, designer-app helper, headless-sdk spec).