fix(llm): enforce reliable structured output

[laipz8200]

Important

1. Make sure you have read our contribution guidelines
2. Search existing issues and pull requests to confirm this change is not a duplicate
3. Open or identify the issue this pull request resolves or advances
4. Use a Conventional Commits title for this pull request, and mark breaking changes with !
5. Remember that the pull request title will become the squash merge commit message
6. If CLA Assistant prompts you, sign CLA.md in the pull request conversation

Related Issue

Closes #236

Summary

• normalize legacy structured-output switches to the canonical structured_output_switch_on field
• validate Draft 7 schema configuration and final blocking, streaming, and polling results with stage and JSON path context
• preserve supported, unsupported, and unknown provider capability states while forwarding merged Slim parameters

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation

CLA Assistant has not prompted me.