Force reasoning mode for GLM-5.3 chat templates (#39227)

Co-authored-by: Brayden Zhong <b8zhong@uwaterloo.ca>
Co-authored-by: Mohammad Angkad <mohammad.angkad@radixark.ai>
Co-authored-by: Xinyuan Tong <xinyuantong.cs@gmail.com>
Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

python/sglang/srt/parser/template_detection.py

@@ -168,6 +168,13 @@ REASONING_MODE_RULES = (
             and not ctx.has_text("thinking")
         ),
     ),
+    DetectionRule(
+        name="glm53_always_think",
+        value=ReasoningToggleConfig(special_case="always"),
+        # GLM-5.3's generation prompt opens <think>, so the output carries no
+        # opening tag; lambda because _is_glm53 is defined below.
+        predicate=lambda ctx: _is_glm53(ctx),
+    ),
     DetectionRule(
         name="mistral_reasoning_effort",
         value=ReasoningToggleConfig(special_case="mistral"),