Add MCP tools for safe schema updates

[ascorbic]

Goal

Agents need to update schemas without deleting and recreating fields or collections.

Scope

Add MCP tools for updating collections and fields, backed by the same validation and safety rules as the admin and REST APIs.

Acceptance criteria

• MCP exposes update collection and update field operations.
• Updates do not require destructive delete-and-recreate workflows.
• Tools enforce RBAC and MCP scopes consistently.
• Error responses make unsafe schema changes clear to agents.
• Tests cover successful updates and rejected unsafe updates.