Add streaming support to XAI provider and update example usage

examples/generate_stream_text.py

@@ -1,9 +1,7 @@
 from _context import sm
 
 # Defaults to the default provider (openai)
-r = sm.generate_text(
-    "Write a poem about the moon", llm_model="gpt-4o-mini", stream=True
-)
+r = sm.generate_text("Write a poem about the moon", llm_provider="xai", stream=True)
 
 for chunk in r:
     print(chunk, end="", flush=True)