Add OpenAI structured output schema support

[aksg87]

Fixes #59

Summary

• Add an OpenAI provider schema that maps examples to OpenAI Chat Completions
  response_format={"type": "json_schema", ...} payloads.
• Wire OpenAI schema constraints through the provider/factory path while keeping
  runtime response_format overrides explicit and warned.
• Enforce JSON-only structured outputs for OpenAI, with clear errors for YAML.
• Add unit and live coverage for OpenAI schema generation, schema application,
  runtime overrides, schema clearing, YAML rejection, and strict structured
  output behavior.

Testing

• tox -e format
• tox -e lint-src
• tox -e lint-tests
• pytest tests/ -ra -m 'not live_api'
• pytest tests/test_kwargs_passthrough.py tests/schema_test.py tests/factory_schema_test.py tests/provider_schema_test.py -q
• tox -e live-api