Fix/showresponse model info optional

[SaiDevaHarshaR]

What this fixes

Fixes #607 — ShowResponse raises ValidationError when cloud models
omit model_info from /api/show response.

Root cause

modelinfo field was declared as Optional but Field(alias='model_info')
had no default value, making it still required by Pydantic.

Fix

Added default=None to the Field definition:

# before
modelinfo: Optional[Mapping[str, Any]] = Field(alias='model_info')

# after  
modelinfo: Optional[Mapping[str, Any]] = Field(default=None, alias='model_info')

Testing

Added test that simulates cloud model response with missing model_info.
Previously raised ValidationError, now returns None correctly.

Closes #607