[Model] Add step-deepresearch tool parser

[randzero]

Purpose

Added native tool parser support for Step DeepResearch model in vLLM.

This PR introduces StepDRToolParser which handles the steptml XML format for tool calls:

<steptml:toolcall>
<steptml:invoke name="function_name">
<steptml:parameter name="param1">value1</steptml:parameter>
</steptml:invoke>
</steptml:toolcall>

Test Plan

python vllm/entrypoints/openai/tool_parsers/stepdr_tool_parser.py

Test Result

--- TEST 1: Non-streaming with array parameter ---
Tools called: True
Content: None
Tool 0: web_search - {"query": ["AI news", "latest developments"], "topk": 10}

**************************

--- TEST 2: Streaming with content before/after ---
Streaming output:
  Content: 'Let'
  Content: ' me'
  Content: ' search'
  Content: ' for'
  Content: ' that'
  Content: ' information'
  Content: '...\n'
  ToolCall[0]: name=web_search
  ToolCall[0]: arguments={"query": ["machine learning", "deep learning"]}
  ToolCall[1]: name=open_url
  ToolCall[1]: arguments={"url": "https://example.com/article"}
  Content: '\n'
  Content: 'Here'
  Content: ' is'
  Content: ' the'
  Content: ' summary'
  Content: '.'

--- TEST 3: Streaming with simple array ---
Streaming output:
  ToolCall[0]: name=web_search
  ToolCall[0]: arguments={"query": ["python", "tutorial", "beginner", "examples"]}

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.