fix: correct border strip test data to avoid false internal gaps

Content word_boxes in test used x-spacing (i%3)*100 which created
internal gaps larger than the border-to-content gap. Changed to
(i%2)*51 so content words overlap and the border gap remains dominant.

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

klausur-service/backend/tests/test_grid_editor_api.py

@@ -1117,7 +1117,9 @@ class TestBorderStripFilter:
         ]
         content_wbs = []
         for i in range(20):
-            content_wbs.append(self._make_wb(f"word{i}", 179 + (i % 3) * 100, 100 + i * 40))
+            # Place content words at x=179 and x=280 (gap=1px between them,
+            # much smaller than the 45px border-to-content gap)
+            content_wbs.append(self._make_wb(f"word{i}", 179 + (i % 2) * 51, 100 + i * 40))
         # Build zone with cells
         cells = []
         # Border-only cells