[executorch][muse] Fix the DFlash final-block draft counts test

CudaDFlashNative.FinalWholeBlockFitsExactlyAndPreservesAllPrefillRows has
failed in unittest-cuda-muse-glimmer-batching since #24043 added draft
forward count checks to it:

  Expected: (drained.draft_forwards - drain.draft_forwards) >
            (drained.static_draft_forwards - drain.static_draft_forwards),
  actual: 1 vs 1

The test expected the decode at position 78 to drain the 79 hidden rows
dynamically before the static draft. But run_target drains every owner
it touches right after appending hidden rows (#24042), so the 78-row
prefill already drains down to the fresh context (32). The decode then
holds at most one block of rows and takes a single static draft, and the
dynamic forwards fall before the window the test measured.

Measure from before the prefill instead: the prefill drains through
dynamic drafts only, and the decode runs exactly one static draft.

Author: Gasoonjia

examples/models/muse-glimmer/tests/cuda_dflash_native_test.cpp

@@ -598,20 +598,23 @@ TEST_F(CudaDFlashNative, FinalWholeBlockFitsExactlyAndPreservesAllPrefillRows) {
   ASSERT_TRUE(executor_);
   const auto sid = open();
   b::BatchOutput out;
+  const auto start = executor_->stats();
   ASSERT_TRUE(executor_->execute({{input(sid, 0, Tokens(78, 1), false)}}, out));
   Peer::check(*executor_, sid, 78);
   EXPECT_EQ(executor_->stats().hidden_rows_d2h, 78);
+  // 78 prefill rows exceed the fresh context, so the prefill itself drains
+  // them through the dynamic draft.
+  const auto prefilled = executor_->stats();
+  EXPECT_GT(prefilled.draft_forwards - start.draft_forwards, 0);
+  EXPECT_EQ(prefilled.static_draft_forwards, start.static_draft_forwards);
   fixture_->rejected.fill(0);
-  const auto drain = executor_->stats();
   ASSERT_TRUE(executor_->execute({{input(sid, 78, {1})}}, out));
   EXPECT_EQ(out.outputs[0]->tokens, predictions(78, 2));
   Peer::check(*executor_, sid, 80);
-  // 79 rows exceed one static block: drain dynamically, then draft statically.
-  const auto drained = executor_->stats();
-  EXPECT_EQ(drained.static_draft_forwards - drain.static_draft_forwards, 1);
-  EXPECT_GT(
-      drained.draft_forwards - drain.draft_forwards,
-      drained.static_draft_forwards - drain.static_draft_forwards);
+  // The drained remainder fits one block: the decode drafts statically.
+  const auto drafted = executor_->stats();
+  EXPECT_EQ(drafted.draft_forwards - prefilled.draft_forwards, 1);
+  EXPECT_EQ(drafted.static_draft_forwards - prefilled.static_draft_forwards, 1);
   fixture_->rejected.fill(15);
   const auto before = executor_->stats();
   ASSERT_TRUE(executor_->execute({{input(sid, 80, {prediction(79)})}}, out));