- Use generate_content_stream() for streaming requests
- Create async generator that yields OpenAI-compatible chunks
- Extract text from each stream chunk
- Generate unique chunk IDs
- Format chunks as chat.completion.chunk objects
- Include delta content in each chunk
- Maintain non-streaming functionality for regular requests

This fixes the streaming issue where Google GenAI was returning
a dict instead of an iterable, causing 'JSONResponse object is
not iterable' errors.