[Bugfix] Guard missing end_token substring in streaming reasoning parsers

[arnavahire19]

Motivation

In BaseThinkingReasoningParser and DeepSeekR1ReasoningParser, when end_token_id is present in delta_token_ids, delta_text.find(self.end_token) is called to split reasoning and content text.

If self.end_token literal string is not present in delta_text (e.g. during multi-token deltas where the special token is detokenized as empty or formatted separately), find() returns -1. Previously, slicing delta_text[:end_index] evaluated to delta_text[:-1], silently clipping the last character off the reasoning buffer and slicing into content.

Proposed Changes

• Added presence check if end_index != -1: before slicing delta_text in BaseThinkingReasoningParser and DeepSeekR1ReasoningParser.
• If end_index == -1, safely keeps reasoning = delta_text without character truncation.
• Added unit test test_end_token_id_without_literal_substring in tests/reasoning/test_base_thinking_reasoning_parser.py covering multi-token delta handling.

[claude]

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