fix(sdk): BUG-015 validate URL in capture() when using ScreenshotOptions

- Add URL validation after options.to_dict() in Python SDK
- Add failing test first (TDD), then fix
- All 17 Python SDK tests passing

sdk/python/src/snapapi/client.py

@@ -144,6 +144,8 @@ class SnapAPI:
         """
         if options:
             body = options.to_dict()
+            if not body.get('url'):
+                raise ValueError("url is required")
         else:
             if not url:
                 raise ValueError("url is required")