[Bug][Spec] fix startup crash and reduce CUDA graph memory usage for speculative adaptive (#35275)

Co-authored-by: alphabetc1 <2508695655@qq.com>
Co-authored-by: Shuwen Wang <47200617+alphabetc1@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/registered/unit/model_executor/test_model_runner_decode_rows.py

@@ -0,0 +1,51 @@
+import tempfile
+import unittest
+from types import SimpleNamespace
+from unittest.mock import patch
+
+from sglang.srt.model_executor.model_runner import ModelRunner
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=1, suite="base-a-test-cpu")
+
+
+class _FakeModelRunner:
+    max_decode_logits_rows = ModelRunner.max_decode_logits_rows
+
+    def __init__(self, *, initial_width: int, cuda_graph_bs: list[int]):
+        self.initial_width = initial_width
+        self.cuda_graph_bs = cuda_graph_bs
+
+    def decode_num_tokens_per_req(self, *, num_draft_tokens=None):
+        return self.initial_width if num_draft_tokens is None else num_draft_tokens
+
+
+def _alignment_8_capture_bs(runner, width):
+    return ([bs for bs in runner.cuda_graph_bs if bs * width % 8 == 0], [])
+
+
+class TestModelRunnerDecodeRows(unittest.TestCase):
+    def test_adaptive_sizing_covers_a_wider_candidate_width(self):
+        """The shared logits buffer is sized for the widest adaptive candidate
+        width: bs 12 at width 6 needs 72 rows."""
+        runner = _FakeModelRunner(initial_width=4, cuda_graph_bs=[4, 8, 12])
+        with tempfile.NamedTemporaryFile("w", suffix=".json") as f:
+            f.write('{"1":{"candidate_steps":[3,5]}}')
+            f.flush()
+            spec = SimpleNamespace(
+                speculative_adaptive=True, speculative_adaptive_config=f.name
+            )
+            with patch(
+                "sglang.srt.model_executor.model_runner.get_spec", return_value=spec
+            ), patch(
+                "sglang.srt.model_executor.model_runner.max_speculative_num_draft_tokens",
+                return_value=6,
+            ), patch(
+                "sglang.srt.model_executor.model_runner.get_batch_sizes_to_capture",
+                side_effect=_alignment_8_capture_bs,
+            ):
+                self.assertEqual(runner.max_decode_logits_rows(), 72)
+
+
+if __name__ == "__main__":
+    unittest.main()