Request: Built-in Anthropic (Claude) and xAI (Grok) providers

## Describe the overall idea and motivation

Add first-class built-in providers so `lx.extract(model_id="claude-sonnet-5")` and `lx.extract(model_id="grok-4.6")` auto-route without LiteLLM or a custom plugin.

Current-generation IDs this would cover:

- Claude: `claude-sonnet-5` (default), `claude-opus-5`, `claude-fable-5`, `claude-haiku-4-5`
- Grok: `grok-4.6` (default), plus `grok-4.5` / `grok-*`

Env keys: `ANTHROPIC_API_KEY`, `XAI_API_KEY` (Grok also uses the existing OpenAI SDK extra at `https://api.x.ai/v1`).

## Related to an issue?

Related to #99 (plugin support) and #187 (LiteLLM, which can already reach Anthropic as a community plugin). This request is for built-in routing of the native model IDs, matching how Gemini / OpenAI / Ollama work today.

## Possible solutions and alternatives

**Proposed:** two optional built-in providers in `langextract.providers`:

- `AnthropicLanguageModel` — Anthropic Messages API
- `XAILanguageModel` — subclass of the OpenAI-compatible provider with xAI base URL + `XAI_API_KEY`

Factory env-key lookup would recognize `claude*` / `grok*` the same way it already recognizes `gemini*` / `gpt*`.

**Alternatives:**

- Keep using `langextract-litellm` (#187) and pass Anthropic/xAI through LiteLLM.
- Publish separate `langextract-anthropic` / `langextract-xai` community plugins (CONTRIBUTING.md's preferred path for new providers).

Built-in support is still useful because `claude-*` and `grok-*` IDs currently match no provider and fail with `InferenceConfigError`.

## Priority and timeline considerations

Nice-to-have / developer-experience. Claude 5 and Grok 4.6 are the current generally-available IDs as of August 2026.

## Additional context

`pyproject.toml` extras (`langextract[anthropic]`) would be a maintainer follow-up if this is accepted; a first patch can document `pip install anthropic` so the PR does not touch protected infrastructure files.

I would like to contribute this and have an implementation ready.

Read CONTRIBUTING.md and inspect langextract.providers plus the factory's existing model-ID and environment-key lookup for Gemini, OpenAI, and Ollama. Confirm the proposed Anthropic Messages API and OpenAI-compatible xAI routes, including the documented pip install requirement. Done means Claude and Grok IDs route without LiteLLM or a plugin and the relevant provider behavior is covered.

Written by the indexing model from the issue text.