Fix stale OpenAI schema support reference

[fallintoplace]

Description

Fixes #59

Type: Documentation

Summary

The OpenAI section in the bundled provider reference predates structured-output
support added in #468 and user-provided schemas added in #483. It still says
that OpenAI has no schema class and that use_schema_constraints does nothing.

This update:

• documents that the provider exposes OpenAISchema and derives constraints
  from examples
• notes that output_schema is supported when the selected model supports
  structured outputs
• recommends leaving fence_output and use_schema_constraints unset so the
  provider can configure them automatically
• keeps the existing JSON-mode fallback and batch-parallelism guidance

Only skills/langextract-usage/references/providers.md changes. No runtime
behavior or public API is modified.

How Has This Been Tested?

The corrected statements were checked against
OpenAILanguageModel.get_schema_class(), OpenAISchema.from_examples(),
OpenAISchema.from_schema_dict(), and the factory defaults.

git diff --check
awk 'length($0) > 80 {print FNR ":" length($0) ":" $0}' skills/langextract-usage/references/providers.md

All required CI checks pass, including formatting, Python 3.10 through 3.12,
and the plugin and Ollama integration suites.

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page and am covered by the CLA check.
• The approach requested in Schema constraints for OpenAI not supported #59 was implemented and accepted in Add OpenAI structured output schema support #468; this change completes its documentation requirement.
• I made the needed documentation correction.
• Existing tests cover the documented behavior; this change does not modify code.
• I followed the repository style guidance. pylint is not applicable to this Markdown-only change, and the formatting check passes.