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/tests/test_snapapi.py

@@ -165,8 +165,12 @@ class TestSnapAPI(unittest.TestCase):
             self.snap.capture(None)
         self.assertEqual(str(cm.exception), "url is required")
 
-        # Note: When using ScreenshotOptions object, URL validation doesn't happen early
-        # This could be considered a bug in the SDK but we're testing current behavior
+    def test_capture_raises_value_error_if_options_has_empty_url(self):
+        """capture(options=ScreenshotOptions(url='')) should raise ValueError."""
+        options = ScreenshotOptions(url="")
+        with self.assertRaises(ValueError) as cm:
+            self.snap.capture(options=options)
+        self.assertEqual(str(cm.exception), "url is required")
 
     @patch('snapapi.client.urllib.request.urlopen')
     def test_capture_raises_snap_api_error_on_http_error(self, mock_urlopen):