Add built-in Anthropic (Claude 5) and xAI (Grok 4.6) providers

[jaideepkumar69-tech]

Description

Add first-class built-in providers so lx.extract(model_id="claude-sonnet-5") and lx.extract(model_id="grok-4.6") auto-route, matching Gemini / OpenAI / Ollama.

• AnthropicLanguageModel — Anthropic Messages API (claude-*). Default claude-sonnet-5; also claude-opus-5, claude-fable-5, claude-haiku-4-5.
• XAILanguageModel — OpenAI-compatible client at https://api.x.ai/v1 (grok-*). Default grok-4.6.
• Factory env lookup: ANTHROPIC_API_KEY / XAI_API_KEY (falls back to LANGEXTRACT_API_KEY).

Does not change pyproject.toml (protected infrastructure). Claude needs pip install anthropic; Grok reuses langextract[openai].

This extends the provider-registry work discussed in the community-supported provider thread.

Related to #99
Related to #522
Related to #187

Feature

How Has This Been Tested?

$ pytest tests/claude_grok_provider_test.py tests/registry_test.py tests/factory_test.py

51 passed. Coverage includes:

• claude-sonnet-5 / claude-opus-5 / claude-fable-5 / claude-haiku-4-5 route to AnthropicLanguageModel
• grok-4.6 / grok-4.5 route to XAILanguageModel
• factory env-key + xAI base URL injection
• Anthropic message content flattening (mocked SDK)

Also ran pyink on the new/changed Python files. CI: format-check and unit tests + pylint on 3.10 / 3.11 / 3.12.

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran pylint over the affected code.