[Bugfix] Clear conflicting structured outputs in strict tool calling

[alexeldeib]

Purpose

ToolParser.adjust_request's strict structural-tag path (added in #40894, gated by VLLM_ENFORCE_STRICT_TOOL_CALLING) installs structural_tag on a pre-existing StructuredOutputsParams via in-place attribute assignment, and returns without nulling response_format. The in-place set bypasses StructuredOutputsParams.__post_init__, so the params retain a prior mutually-exclusive constraint (json/regex/choice/grammar/json_object, or one lowered from response_format) alongside the new structural_tag. On the next re-validation this trips the one-constraint invariant:

ValueError: You can only use one kind of structured outputs constraint but multiple are specified

So a strict-mode request that also carries a structured-output constraint or a response_format returns HTTP 400 when it should succeed. This affects any parser that installs a structural tag — currently DeepSeek-V4 and Qwen3-Coder via get_structural_tag. The env var is off by default; requests with no pre-existing constraint are unaffected.

Fix: rebuild structured_outputs with only the structural tag (preserving the whitespace / additional-properties knobs) and null response_format, mirroring Step 2 of the same method. The same "tool constraint wins, response_format dropped" resolution already exists in Step 2 and the DeepSeek-V3.2 override (#41178), and is the intent of the open auto-path fix #39969.

Test Plan

pytest tests/tool_use/test_strict_tool_calling_adjust_request.py

Added CPU-only tests (no model required): enable strict mode, give a parser a structural tag, and send tools together with a response_format or a structured_outputs.json constraint, for both tool_choice="auto" and tool_choice="required".

Test Result

• Before: adjust_request leaves two constraints; to_sampling_params raises the ValueError above. The conflict tests fail.
• After: structured_outputs holds only the structural tag, response_format is None, the caller's whitespace knobs are preserved. All tests pass; the no-pre-existing-constraint case passes either way.

Equivalently over HTTP, with strict mode on: a tool_choice="auto" request that also sets response_format returns 400 before this change and a normal tool call after; a required-tool request is unaffected (that path already rebuilds).

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR (origin: feat: update xgrammar==0.2.0 to use structural tags for strict tool calling + reasoning for more models #40894)
• The test plan (pytest command above)
• The test results (before/after)
• (Optional) Documentation update — n/a (bugfix; no API/model surface change)