[Frontend] Fix reasoning_tokens for text-based parsers in Responses API (#33513)

Signed-off-by: Jaeyeon Kim <anencore94@gmail.com>

tests/reasoning/test_base_thinking_reasoning_parser.py

@@ -167,6 +167,23 @@ class TestBaseThinkingReasoningParserMethods:
             is False
         )
 
+    def test_count_reasoning_tokens(self, test_tokenizer):
+        """Count tokens between start/end markers."""
+        parser = TestThinkingReasoningParser(test_tokenizer)
+        start = parser.start_token_id
+        end = parser.end_token_id
+        token_ids = [0, start, 11, 12, end, 99]
+        assert parser.count_reasoning_tokens(token_ids) == 2
+
+    def test_count_reasoning_tokens_nested(self, test_tokenizer):
+        """Ensure nested thinking spans count all inner tokens safely."""
+        parser = TestThinkingReasoningParser(test_tokenizer)
+        s = parser.start_token_id
+        e = parser.end_token_id
+        token_ids = [s, 1, s, 2, e, 3, e]
+        # Tokens 1,2,3 are inside reasoning (depth>0) => 3 tokens
+        assert parser.count_reasoning_tokens(token_ids) == 3
+
     def test_extract_content_ids(self, test_tokenizer):
         """Test the extract_content_ids method."""
         parser = TestThinkingReasoningParser(test_tokenizer)