fix: make model_info optional in ShowResponse for cloud model compatibility

[CyberRaccoonTeam]

Summary

ShowResponse raises a ValidationError when model_info is omitted by cloud model responses (e.g., when using OpenAI-compatible endpoints through Ollama).

Problem

# This fails with ValidationError
response = client.show("gemma3:4b")
# ValidationError: Key "model_info" not found

Cloud models and some local models do not return model_info in their response, causing a validation error instead of gracefully handling the missing field.

Solution

Make model_info optional with a default of None in the ShowResponse model.

Testing

• ✅ Tested with local models that return model_info
• ✅ Tested with cloud models that omit model_info
• ✅ Existing tests pass

Fixes the issue where client.show() crashes on cloud model responses.