[CXP-shawn]

Summary

Follow-up hardening for #6889 and the MCP Server Action vulnerability reported in #6757.

• require a valid deployment CODE for every MCP Server Action when MCP is enabled
• fail closed when ENABLE_MCP is true but CODE is not configured
• pass the configured access code from all MCP UI and chat callers
• stop MCP child processes from inheriting the complete application environment; only the SDK safe defaults and explicitly configured MCP variables are passed
• document the MCP CODE requirement in English, Chinese, and Korean

Why

#6889 prevents MCP actions from running when ENABLE_MCP is disabled. An MCP-enabled deployment would still expose privileged Server Actions without authentication, and spawned MCP processes inherited unrelated application secrets. This PR closes both gaps.

Verification

• yarn test:ci test/mcp-actions-require-enabled.test.ts test/mcp-client-environment.test.ts
• yarn build

Both completed successfully.