feat: Add guided decoding support to OpenAI frontend

[pei0033]

What does the PR do?

This PR adds comprehensive guided decoding support to the OpenAI frontend, enabling users to constrain model outputs to specific formats through the OpenAI-compatible API.
The implementation supports both vLLM and TensorRT-LLM backends with multiple guide types including JSON schema, regex patterns, choice-based selection, and EBNF grammar.

Checklist

• I have read the Contribution guidelines and signed the Contributor License
  Agreement
• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• I ran pre-commit locally (pre-commit install, pre-commit run --all)
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

N/A

Where should the reviewer start?

Please focus on these key files:

1. python/openai/openai_frontend/schemas/openai.py - Review the new schema fields guided_decoding_guide_type and guided_decoding_guide added to both completion request models
2. python/openai/openai_frontend/engine/utils/triton.py - Check the implementation of guided decoding integration for both vLLM and TensorRT-LLM backends
3. python/openai/README.md - Verify the comprehensive documentation and examples for different guide types

Test plan:

please follow the codes in README.md

Caveats:

1. Different usage patterns per backend:
2. Guided decoding may not function properly when used in conjunction with tool calling
3. Currently relies on backend validation; frontend doesn't validate guide format compatibility

Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)