[Bugfix][Frontend] Attach structural tag for strict tool calling when parsers share engine

[Alphaxiaoteng]

Description

Fixes #53745.

Root Cause

When the reasoning parser and tool parser resolve to the same ParserEngine subclass (e.g. --reasoning-parser qwen3 --tool-call-parser qwen3_coder), ParserManager.get_parser returned the underlying ParserEngine class directly. This caused two problems:

1. The returned class did not retain the registered tool_parser_cls (e.g. Qwen3EngineToolParser which carries structural_tag_model = "qwen_3_coder"), instead retaining the generic Qwen3ParserToolAdapter with no structural_tag_model.
2. ParserEngine.adjust_request only set request.skip_special_tokens = False and did not call into _apply_structural_tag or the tool parser's adjust_request.

Consequently, strict tool calling with shared parser engines produced no xgrammar structural tags, resulting in unconstrained tool-call arguments.

Solution

1. In ParserManager.get_parser, when reasoning and tool engines match, construct a dynamic subclass retaining reasoning_parser_cls and tool_parser_cls.
2. Move _apply_structural_tag to Parser so that both DelegatingParser and ParserEngine can use it.
3. In ParserEngine.adjust_request, invoke _apply_structural_tag and ToolParser.adjust_request using self.tool_parser_cls.

Test Plan

• Added test_parser_manager_shared_engine_strict_tool_calling in tests/parser/engine/test_parser_engine.py verifying that structural tags are attached under strict tool calling with shared engines.
• Verified unit test suite: pytest tests/parser/engine/ tests/tool_parsers/test_qwen3coder_tool_parser.py tests/tool_parsers/test_deepseekv4_tool_parser.py (3,863 tests passed).
• Verified linter: ruff check on changed files passed.

[claude]

Claude Code Review

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Alphaxiaoteng.

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