fix(reasoning): let --enable-strict-thinking works for DeepSeek-V4 (#32400)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Xinyuan Tong <xinyuantong.cs@gmail.com>

test/registered/unit/parser/test_reasoning_parser.py

@@ -6,6 +6,7 @@ from sglang.srt.parser.reasoning_parser import (
     Apertus2509Detector,
     BaseReasoningFormatDetector,
     DeepSeekR1Detector,
+    DeepSeekV4Detector,
     Gemma4Detector,
     Glm45Detector,
     HunyuanDetector,
@@ -167,6 +168,21 @@ class TestQwen3Detector(CustomTestCase):
         self.assertEqual(result.reasoning_text, "")
 
 
+class TestDeepSeekV4Detector(CustomTestCase):
+    def test_strict_thinking_excludes_deepseek_control_tokens(self):
+        detector = ReasoningParser(model_type="deepseek-v4").detector
+        self.assertIsInstance(detector, DeepSeekV4Detector)
+        self.assertEqual(
+            detector.think_excluded_tokens,
+            ["<｜end▁of▁sentence｜>", "｜DSML｜"],
+        )
+
+    def test_thinking_stays_explicit_opt_in(self):
+        detector = ReasoningParser(model_type="deepseek-v4").detector
+        self.assertEqual(detector.reasoning_default, "explicit_thinking")
+        self.assertTrue(detector.thinks_internally)
+
+
 class TestInklingDetector(CustomTestCase):
     def test_streaming_routes_blocks_across_all_string_boundaries(self):
         detector = InklingDetector()