Replace urljoin with string concatenation for base URL construction

urljoin(base, path) silently drops any path prefix from the base URL
when path starts with '/', so base_vws_url values like
'http://localhost/mock' would have '/mock' ignored. String concatenation
preserves the full base URL path as intended.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: adamtheturtle

src/vws/_vws_request.py

@@ -2,8 +2,6 @@
 API.
 """
 
-from urllib.parse import urljoin
-
 import requests
 from beartype import BeartypeConf, beartype
 from vws_auth_tools import authorization_header, rfc_1123_date
@@ -64,7 +62,7 @@ def target_api_request(
         **extra_headers,
     }
 
-    url = urljoin(base=base_vws_url, url=request_path)
+    url = base_vws_url.rstrip("/") + request_path
 
     requests_response = requests.request(
         method=method,