fix: resolve MCP tool validation errors in beta server

[kiratchhina]

Summary

This PR fixes two validation errors in the beta MCP server that were causing tool failures in production.

Issue 1: Site Publishing Fails with customDomains Validation Error

Production Error:

{
  "message": "Validation Error: [\"Value (customDomains)'s type should be array\"]",
  "code": "validation_error",
  "statusCode": 400
}

Event ID: evt_3BlCAWmNUYSYB94s8VTWyvCJ5d0
Actor: kelly@vector.co
Timestamp: Apr 1, 2026, 10:35:26 AM

Root Cause:
Incorrect Zod chaining order in sites.ts. When .default([]) comes before .optional(), the field can still be undefined, causing the Webflow API to reject the request.

Before:

customDomains: z
  .array(z.string())
  .default([])      // ❌ Wrong order
  .optional()

After:

customDomains: z
  .array(z.string())
  .optional()       // ✅ Correct order
  .default([])

Why this works:
In Zod, .optional().default([]) ensures that if the field is omitted, it defaults to []. With the original order, the field could be undefined even with .default([]), which caused the API validation to fail.

---

Issue 2: Empty Error Messages in element_snapshot_tool

Production Error:

{
  "name": "Error",
  "message": "",
  "error": {}
}

Event ID: evt_3BpOqtX0Z2ZbizQ06We4pzL88rg
Resource: element_snapshot_tool
Timestamp: Apr 2, 2026, 10:18:58 PM

Root Cause:
When element_snapshot_tool's RPC call fails and returns an empty message, the error handler creates new Error(""), resulting in unhelpful debugging information.

Before:

return formatErrorResponse(new Error(message)); // ❌ Empty message

After:

return formatErrorResponse(
  new Error(
    message ||
    `Element snapshot failed with status: ${status}. Response: ${JSON.stringify({ status, message, data })}`
  )
);

Why this works:
Provides a fallback error message with full response details when the RPC call returns an empty message, making debugging significantly easier.

---

Testing

• ✅ Changes applied to beta server only (mcp-server-beta)
• ✅ Stable server unchanged for safety
• ✅ Both fixes address real production errors with documented event IDs

Files Changed

• mcp-server-beta/src/tools/sites.ts - Fixed customDomains Zod chaining
• mcp-server-beta/src/tools/deElement.ts - Fixed empty error messages

Next Steps

After validation in beta:

1. Monitor for recurrence of these errors
2. If successful, apply to stable server
3. Consider auditing other tools for similar patterns