[Feature] Support structural tag templates.

[Seven-Streams]

This PR supports the structural tag templates, provides a method to generate a grammar from a structural tag template and the given values, and provides built-in structural tag templates for several models' tool-calling formats.

[Copilot]

Pull Request Overview

This PR introduces support for structural tag templates, enabling users to generate grammars from templates with dynamic values. The feature includes a template placeholder system, built-in templates for popular models' tool-calling formats (Llama, Kimi, Deepseek, Qwen, Harmony), and comprehensive test coverage.

• Adds template placeholder syntax {placeholder_name[].field} for dynamic value substitution
• Implements built-in structural tag templates for 6 model formats
• Adds extensive test coverage for template expansion and validation

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/python/test_structural_tag_converter.py	Adds 1600+ lines of comprehensive tests for template functionality including const string, JSON schema, regex, grammar, and builtin templates
python/xgrammar/structural_tag.py	Implements template registry system and 6 builtin template functions for model-specific tool-calling formats
python/xgrammar/grammar.py	Adds apply_structural_tag_template method to Grammar class for applying templates with values
cpp/structural_tag.h	Adds C++ API declaration for template application with ToString methods for Format types
cpp/structural_tag.cc	Implements core template detection, placeholder replacement, and format expansion logic (~1100 lines)
cpp/nanobind/nanobind.cc	Adds Python binding for apply_structural_tag_template with kwargs handling
cpp/nanobind/python_methods.h	Adds necessary includes for template functionality
cpp/nanobind/python_methods.cc	Adds include for structural_tag.h
cpp/json_schema_converter.cc	Enhances enum handling to support both JSON and XML formats with proper quote stripping

Comments suppressed due to low confidence (1)

python/xgrammar/grammar.py:450

• Nested for statement uses loop variable 'key' of enclosing for statement.

                for key, value in item.items():

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