Add regression test for SAME_UPPER stride>1 padding (#26734)

Add test case Conv2D_AutoPad_SAME_UPPER_Stride2 that verifies correct
asymmetric padding calculation when auto_pad=SAME_UPPER and stride>1.

This catches the off-by-one padding bug fixed in this PR.

Author: ingyukoh

onnxruntime/test/providers/cpu/nn/conv_op_test.cc

@@ -502,6 +502,49 @@ TEST(ConvTest, Conv2D_AutoPad1) {
   TestConvOp(attrs, {X, W}, {X_shape, W_shape}, expected_vals, Y_shape, true);
 }
 
+// Regression test for issue #26734: SAME_UPPER with stride > 1
+// Tests asymmetric padding calculation that was incorrect in WebGPU EP
+TEST(ConvTest, Conv2D_AutoPad_SAME_UPPER_Stride2) {
+  ConvOpAndTestAttributes attrs = {
+      "SAME_UPPER",           // auto_pad
+      vector<int64_t>{1, 1},  // dilations
+      1,                      // group
+      vector<int64_t>{3, 3},  // kernel_shape
+      {},                     // pads
+      vector<int64_t>{2, 2},  // strides > 1 triggers asymmetric padding
+      {}                      // excluded EPs
+  };
+
+  // 1x1x4x4 input
+  vector<float> X = {1.0f, 2.0f, 3.0f, 4.0f,
+                     5.0f, 6.0f, 7.0f, 8.0f,
+                     9.0f, 10.0f, 11.0f, 12.0f,
+                     13.0f, 14.0f, 15.0f, 16.0f};
+  vector<int64_t> X_shape = {1, 1, 4, 4};
+
+  // 3x3 kernel of all 1s for easy verification
+  vector<float> W = {1.0f, 1.0f, 1.0f,
+                     1.0f, 1.0f, 1.0f,
+                     1.0f, 1.0f, 1.0f};
+  vector<int64_t> W_shape = {1, 1, 3, 3};
+
+  // Output: 2x2 (ceil(4/2) = 2)
+  // SAME_UPPER with total_pad=1: pad_head=0, pad_tail=1
+  vector<int64_t> Y_shape = {1, 1, 2, 2};
+
+  // Expected values:
+  // (0,0): 1+2+3+5+6+7+9+10+11 = 54
+  // (0,1): 3+4+0+7+8+0+11+12+0 = 45
+  // (1,0): 9+10+11+13+14+15+0+0+0 = 72
+  // (1,1): 11+12+0+15+16+0+0+0+0 = 54
+  auto expected_vals = {54.0f, 45.0f, 72.0f, 54.0f};
+
+  TestConvOp(attrs, {X, W}, {X_shape, W_shape}, expected_vals, Y_shape);
+
+  // NNAPI/CoreML EP requires weight to be an initializer
+  TestConvOp(attrs, {X, W}, {X_shape, W_shape}, expected_vals, Y_shape, true);
+}
+
 TEST(ConvTest, Conv2D_AutoPad2) {
   ConvOpAndTestAttributes attrs = {
       "SAME_LOWER",           // auto_pad