[Bugfix] Fix step3p5 parser when using mtp

[mariohong128]

Purpose

Fix step3.5 parser when using mtp.
If model outputs </tool_call><tool_call>< (using mtp will greatly increase the possibility of this), parser will start a new empty toolcall incorrectly.

{\"error\":{\"message\":\"1 validation error for ValidatorIterator\\\\n1.function.name\\\\n  Field required [type=missing, input_value={\\'arguments\\': \\'{}\\'}, input_type=dict]\\\\n    For further information visit https://errors.pydantic.dev/2.11/v/missing None\",\"type\":\"BadRequestError\",\"param\":null,\"code\":400}}

Test Plan

pytest tests/tool_parsers/test_step3p5_tool_parser.py

Test Result

All tests passed.

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[gemini-code-assist]

Code Review

This pull request aims to fix the step3.5 tool parser for scenarios involving multiple tool calls by introducing a fallback_call_id. This new variable is intended to ensure that fallback logic for closing tags applies to the correct tool call, especially in streaming mode. While the overall direction is correct, the implementation for determining fallback_call_id is overly complex and misses a key transition case, which could lead to parsing failures. I've provided a critical review comment with a suggested simplification that makes the logic more robust and corrects the identified issue.

[mariohong128]

@chaunceyjiang We have internally verified the correctness of this parser modification. Can we merge it now?

[chaunceyjiang]

Thanks~