Add Grok 4 Fast and Grok 4.1 Fast, plus xAI native tool calling

[mrubens]

Important

Add new Grok 4 Fast models and enhance xAI native tool calling with tests.

• Models:
  • Add grok-4-fast-reasoning, grok-4-fast-non-reasoning, grok-4-1-fast-reasoning, and grok-4-1-fast-non-reasoning to xaiModels in xai.ts.
  • All new models support native tools, images, and have a 2M context window.
• Tool Calling:
  • Add supportsNativeTools to existing models in xai.ts.
  • Update createMessage in xai.ts to handle native tool calling with tool_choice and parallel_tool_calls.
  • Add tests for native tool calling in xai.spec.ts.
• Tests:
  • Add tests for tool inclusion, tool_choice, and parallel_tool_calls in xai.spec.ts.
  • Test tool_call_partial chunk streaming in xai.spec.ts.

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[roomote]

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Review complete - no issues found.

The PR successfully adds four new Grok 4 Fast models and implements native tool calling support for xAI. All changes follow established patterns, include comprehensive test coverage, and don't introduce any bugs or breaking changes.

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