OrcaRouter provider plugin (langextract-provider-orcarouter)

### Describe the feature

Add an [OrcaRouter](https://www.orcarouter.ai) provider plugin to the community registry. OrcaRouter is an OpenAI-compatible gateway that fronts models from multiple vendors through a single endpoint at `https://api.orcarouter.ai/v1`. It also runs gateway-level, zero-trust security for AI agents on the same endpoint — screening every prompt/response and governing every tool call on a default-deny basis, with no application code changes.

The plugin is implemented as an external LangExtract provider package (the recommended route from `langextract/providers/README.md`). It subclasses `OpenAILanguageModel`, so it inherits Chat Completions, JSON mode, structured outputs, and Batch API support. Model IDs starting with `orcarouter/` (e.g. `orcarouter/auto`) route automatically.

- Package: `langextract-provider-orcarouter`
- Repo: https://github.com/JinhaoSong322/langextract-orcarouter
- Usage: `lx.extract(..., model_id="orcarouter/auto", api_key=os.environ["ORCAROUTER_API_KEY"])`
- Env keys: `ORCAROUTER_API_KEY`, `ORCAROUTER_BASE_URL` (default `https://api.orcarouter.ai/v1`)

Disclosure: I'm an engineer on the OrcaRouter team.

Read langextract/providers/README.md first to confirm the recommended external-provider registration path, then inspect the community registry entry point for how packages are listed. Verify that langextract-provider-orcarouter, its orcarouter/ model IDs, and the documented environment keys appear correctly; no test file is named in the issue.

Written by the indexing model from the issue text.