fix: resolve MCP tool validation errors in beta server

## Issue 1: Site publishing fails with customDomains validation error

**Error:**
```
"message": "Validation Error: [\"Value (customDomains)'s type should be array\"]",
"code": "validation_error"
```

**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.

**Fix:**
Changed customDomains schema from:
```typescript
z.array(z.string()).default([]).optional()
```
To:
```typescript
z.array(z.string()).optional().default([])
```

This ensures customDomains always defaults to an empty array instead of
potentially being undefined.

**Files changed:**
- mcp-server-beta/src/tools/sites.ts

## Issue 2: Empty error messages in element_snapshot_tool

**Error:**
```json
{
  "name": "Error",
  "message": "",
  "error": {}
}
```

**Root cause:**
When element_snapshot_tool's RPC call fails with an empty message, the error
handler creates `new Error("")`, resulting in unhelpful empty error messages.

**Fix:**
Added fallback error message with response details:
```typescript
new Error(
  message ||
  \`Element snapshot failed with status: \${status}. Response: \${JSON.stringify({ status, message, data })}\`
)
```

**Files changed:**
- mcp-server-beta/src/tools/deElement.ts

**Testing:**
These fixes target the beta server only for validation before rolling out
to stable.

Author: kiratchhina

src/tools/deElement.ts

@@ -539,7 +539,12 @@ export const registerDEElementTools = (server: McpServer, rpc: RPCType) => {
             ],
           };
         }
-        return formatErrorResponse(new Error(message));
+        return formatErrorResponse(
+          new Error(
+            message ||
+            `Element snapshot failed with status: ${status}. Response: ${JSON.stringify({ status, message, data })}`
+          )
+        );
       } catch (error) {
         return formatErrorResponse(error);
       }

src/tools/sites.ts

@@ -79,8 +79,8 @@ export function registerSiteTools(
                     .describe("Unique identifier for the site."),
                   customDomains: z
                     .array(z.string())
-                    .default([])
                     .optional()
+                    .default([])
                     .describe(
                       "Array of custom domains to publish the site to.",
                     ),