[Refactor] Consolidate required/named tool_choice streaming into DelegatingParser

[sfeng33]

Purpose

For chat completion api, migrate required/named tool_choice streaming to use parse_delta in the unified parser.

Test Plan

Scenario	tool_choice	Result
Required streaming	"required"	Name + id emitted once, args streamed incrementally, finish_reason="tool_calls"
Named streaming	{"type":"function","function":{"name":"get_weather"}}	Name + id emitted once on first tool_calls chunk, args streamed incrementally
Auto streaming (regression)	"auto"	Tool call streamed normally via existing auto path

Server:
vllm serve Qwen/Qwen3-0.6B \
    --enable-auto-tool-choice \
    --tool-call-parser hermes \
    --chat-template examples/tool_chat_template_hermes.jinja

Sample request body:
{
  "model": "Qwen/Qwen3-0.6B",
  "stream": true,
  "tool_choice": "required",
  "messages": [{"role":"user","content":"What is the weather in Paris?"}],
  "tools": [{"type":"function","function":{
    "name":"get_weather","description":"Get weather",
    "parameters":{"type":"object",
      "properties":{"city":{"type":"string","description":"city name"}},
      "required":["city"]}
  }}]
}

[gemini-code-assist]

Code Review

This pull request refactors the tool call streaming logic by consolidating the handling of 'required' and 'named' tool choices into the abstract parser. It removes redundant extraction logic from the OpenAIServingChat class and ensures that the parser state is correctly initialized with tool call metadata. A potential issue was identified in how the tool call counter is incremented, as the current implementation only checks the first tool call in a delta message, which could lead to inaccuracies if multiple tool calls are initiated simultaneously.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[chaunceyjiang]

LGTM.

One small question: why does your test command explicitly specify an additional chat template?

[sfeng33]

One small question: why does your test command explicitly specify an additional chat template?

Oh it works without specifying the chat template, the agent picked it up this time randomly