[XGrammar] Add structural tag support and example

[akaashrp]

No description provided.

[Copilot]

Pull request overview

This PR adds “structural tag” as a new response_format mode backed by @mlc-ai/web-xgrammar, including request validation, pipeline support, and a new browser example demonstrating MCP-style <tool_call> blocks.

Changes:

• Extend ResponseFormat to support type: "structural_tag" and validate its required fields.
• Update LLMChatPipeline to compile/apply structural-tag constraints via XGrammar.
• Add a new examples/structural-tag-tool-use demo and bump @mlc-ai/web-xgrammar to 0.1.27.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/openai_api_protocols/chat_completion.ts	Adds structural-tag type to ResponseFormat and enforces request invariants in postInitAndCheckFields.
src/llm_chat.ts	Adds structural-tag handling for constrained decoding, plus caching refactor for matcher reuse.
src/error.ts	Introduces InvalidResponseFormatStructuralTagError.
package.json	Bumps @mlc-ai/web-xgrammar to 0.1.27.
package-lock.json	Updates lockfile to reflect new xgrammar version (and refreshed transitive versions).
cleanup-index-js.sh	Adds an additional post-build rewrite to make Parcel packaging work with scriptDirectory initialization.
examples/structural-tag-tool-use/src/mcp_structural_tag.ts	New demo that requests structural-tag constrained tool calls and runs stub tools.
examples/structural-tag-tool-use/src/mcp_structural_tag.html	Minimal demo page wiring up the example.
examples/structural-tag-tool-use/package.json	Parcel-based build/run config for the new example.
examples/structural-tag-tool-use/README.md	Usage instructions and explanation for the example.

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[CharlieFRuan]

LGTM!