Fix Qwen3-Coder required tool parsing

[pst2154]

Summary

Fix qwen3_coder tool parsing for tool_choice="required" / named tool choices when the model emits Qwen3 XML tool-call syntax after reasoning output.

qwen3_coder currently advertises supports_required_and_named for the generic required-tool path. That path expects JSON tool-call output, but Qwen3-Coder emits XML like:

</think>
<tool_call>
<function=get_weather>
<parameter=location>
San Francisco, CA
</parameter>
</function>
</tool_call>

As a result, the reasoning parser can split out the reasoning text, but the required tool call is dropped and the response can return finish_reason="tool_calls" with tool_calls=[].

This PR:

• routes required/named tool choices for qwen3_coder through its XML parser by setting supports_required_and_named = False
• avoids reporting finish_reason="tool_calls" for required tool choice unless a tool call was actually parsed
• adds a regression test for Qwen3 XML tool calls after </think> with tool_choice="required"

Duplicate-work check

Checked open PR searches for:

• qwen3 coder required tool parser
• qwen3 tool_choice required
• qwen3_coder tool_call XML

Related open PRs exist, especially #35936, which implements a broader serving-layer fallback for non-JSON tool_choice="required" in streaming and non-streaming paths. This PR is a narrower alternative focused on qwen3_coder parser capability routing plus the guard that prevents finish_reason="tool_calls" when no tool call was parsed. Other related Qwen reasoning/XML parser PRs (#39055, #44141) address tool calls embedded inside reasoning rather than this required/named tool-choice path.

Validation

• .venv/bin/python -m py_compile vllm/tool_parsers/qwen3coder_tool_parser.py vllm/entrypoints/openai/chat_completion/serving.py tests/tool_parsers/test_qwen3coder_tool_parser.py - passed
• uvx --from ruff ruff check vllm/tool_parsers/qwen3coder_tool_parser.py vllm/entrypoints/openai/chat_completion/serving.py tests/tool_parsers/test_qwen3coder_tool_parser.py - passed
• uv run --with ruff ruff check ... - attempted, but local dependency resolution failed on macOS because the project environment requires unavailable torch==2.11.0+cpu for a resolved split
• Manually validated against a patched vLLM 0.22.0 Ultra NVFP4 + MTP5 deployment: tool_choice="required" weather requests now return get_weather tool calls instead of empty tool_calls.

AI assistance

AI assistance was used to inspect the PR comments/checks, amend the DCO signoff, run targeted local validation, and update this PR description. The human submitter is responsible for reviewing and defending the change end-to-end.

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