fix(dsa): fail fast on fp8_e4m3 KV with tilelang DSA backend on CUDA (#31346)

Co-authored-by: mosya415 <263250241+mosya415@users.noreply.github.com>

test/registered/unit/test_dsa_tilelang_fp8_validation.py

@@ -0,0 +1,41 @@
+"""Rejecting an fp8_e4m3 KV cache with the tilelang DSA backend on CUDA.
+
+Regression: the combination used to boot the server and crash at decode
+CUDA-graph capture with ``kernel main input KV dtype expected bfloat16,
+but got float8_e4m3fn``.
+"""
+
+import unittest
+
+from sglang.srt.arg_groups.overrides import _check_tilelang_dsa_fp8_kv
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=10, suite="base-a-test-cpu")
+
+
+class TestDsaTilelangFp8Validation(CustomTestCase):
+
+    def test_cuda_fp8_tilelang_decode_rejected(self):
+        with self.assertRaises(ValueError):
+            _check_tilelang_dsa_fp8_kv("fp8_e4m3", "flashmla_kv", "tilelang", hip=False)
+
+    def test_cuda_fp8_tilelang_prefill_rejected(self):
+        with self.assertRaises(ValueError):
+            _check_tilelang_dsa_fp8_kv("fp8_e4m3", "tilelang", "trtllm", hip=False)
+
+    def test_hip_fp8_tilelang_allowed(self):
+        # ROCm has a real fp8 tilelang kernel
+        _check_tilelang_dsa_fp8_kv("fp8_e4m3", "tilelang", "tilelang", hip=True)
+
+    def test_bf16_tilelang_allowed(self):
+        # what the CUDA kernel expects
+        _check_tilelang_dsa_fp8_kv("bfloat16", "tilelang", "tilelang", hip=False)
+
+    def test_cuda_fp8_non_tilelang_allowed(self):
+        # fp8-capable backends must pass
+        _check_tilelang_dsa_fp8_kv("fp8_e4m3", "flashmla_kv", "trtllm", hip=False)
+
+
+if __name__ == "__main__":
+    unittest.main()