Add streaming support for xAI provider

[jxnl]

Description

The xAI provider currently supports synchronous and asynchronous operations with both JSON and TOOLS modes, but does not yet support streaming responses (create_iterable and create_partial).

Background

xAI's SDK has a different streaming API compared to other providers. Instead of passing stream=True as a parameter, xAI uses a .stream() method on the chat object that returns tuples of (response, chunk).

Example from xAI docs:

for response, chunk in chat.stream():
    print(chunk.content, end="", flush=True)

What needs to be done

1. Implement streaming support in instructor/client_xai.py for both sync and async clients
2. Handle the conversion from xAI's streaming format to the format expected by instructor's streaming infrastructure
3. Add comprehensive tests in tests/llm/test_xai/test_stream.py covering:
   • create_iterable for multiple object extraction
   • create_partial for partial object streaming
   • Both JSON and TOOLS modes
   • Both sync and async operations

Technical considerations

• The implementation needs to yield response chunks in a format compatible with instructor's process_response function
• For JSON mode: chunks should be extracted from the content
• For TOOLS mode: chunks need to be extracted from tool calls
• The streaming generator should yield chunks as they arrive, not accumulate all content first

References

• xAI Streaming Documentation
• Current xAI implementation: instructor/client_xai.py
• Example streaming implementations: See OpenAI or Anthropic providers

This would be a great first contribution for someone familiar with async generators and streaming APIs!