Exclude already-existing layers from pull progress calculation

Layers that already exist locally should not count towards download
progress since there's nothing to download for them. Only layers that
need pulling are included in the progress calculation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/docker/test_pull_progress.py

@@ -591,11 +591,9 @@ class TestImagePullProgress:
             )
         )
 
-        # Count-based: 2 layers total, each = 50%
-        # cached: 100% (already exists)
+        # Only 1 layer needs pulling (cached layer excluded)
         # pulled: 35% (50% of 70% download weight)
-        # Total: (100 + 35) / 2 = 67.5%
-        assert progress.calculate_progress() == pytest.approx(67.5)
+        assert progress.calculate_progress() == pytest.approx(35.0)
 
         # Complete the pulled layer
         progress.process_event(