Expose structured output support for chat models

[slegarraga]

Summary

• add a supportsStructuredOutputs provider option
• forward the option to OpenAI-compatible chat models
• add a regression test for JSON schema responseFormat without the unsupported responseFormat warning

Fixes #5

Validation

• npm test
• npm run build

Note: npm run lint currently fails locally because eslint is not installed after npm ci (sh: eslint: command not found).