Refactor URL validation to use a constant for allowed schemes

lib/galaxy/webapps/galaxy/api/proxy.py

@@ -35,13 +35,15 @@ URLQueryParam: str = Query(
     description="The URL of the remote file",
 )
 
+ALLOWED_SCHEMES = ("https", "http")
+
 
 def is_valid_url(url: str) -> bool:
     if url.count("://") != 1:
         return False  # Ensure there is exactly one scheme
     try:
         parsed = urlparse(url)
-        return all([parsed.scheme in ("http", "https"), parsed.netloc])
+        return all([parsed.scheme in ALLOWED_SCHEMES, parsed.netloc])
     except ValueError:
         return False