Harvest-API: `POST /api/validate` with `fetch_method=url` returns 500 with HTML page

**Description**
The /api/validate endpoint returns HTTP 500 with an HTML error page when fetch_method=url is used. The paste method works correctly.

**Steps to Reproduce**
POST [https://harvest.data.gov/api/validate Content-Type](https://harvest.data.gov/api/validateContent-Type): application/json

{ "fetch_method": "url", "schema": "dcatus1.1: federal dataset", "url": "https://www.gsa.gov/data.json" }

**Expected Behavior**
JSON response containing validation results.

**Actual Behavior**
HTTP 500 with an HTML "Site unavailable" page.

**Notes**
Confirmed April 27, 2026. The paste method (fetch_method=url replaced with fetch_method=paste and json_text provided) works correctly and returns {"validation_errors": [...]}. Only the URL fetch method is broken. This is the primary intended use case for agencies validating their data.json before harvest -- high priority fix. Needs testing with additional agency URLs to rule out timeout issues specific to [gsa.gov](http://gsa.gov/).