Fix test endpoints reporting pre-filter image dimensions

- WebSocket test: move w,h capture to after PP filter application
  so downscaler effect is reflected in reported resolution
- HTTP test: read actual thumbnail dimensions from filtered image
  instead of using pre-computed values

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

server/src/wled_controller/api/routes/_test_helpers.py

@@ -188,12 +188,13 @@ async def stream_capture_test(
         tc = total_capture_time
         fps = fc / elapsed if elapsed > 0 else 0
         avg_ms = (tc / fc * 1000) if fc > 0 else 0
-        w, h = final_frame.size
 
         # Apply PP filters to final images
         if pp_filters:
             final_frame = _apply_pp_filters(final_frame, pp_filters)
 
+        w, h = final_frame.size
+
         full_uri = _encode_jpeg(final_frame, FINAL_JPEG_QUALITY)
         thumb = _make_thumbnail(final_frame, FINAL_THUMBNAIL_WIDTH)
         thumb_uri = _encode_jpeg(thumb, 85)