feat: add backward compatibility for models.get("owner/name") format

[zeke]

This PR adds backward compatibility for the legacy models.get("owner/name") format while maintaining full compatibility with the new models.get(model_owner="owner", model_name="name") format.

Changes

• Modified the get() method in both sync and async ModelsResource classes to accept an optional positional argument for legacy format
• Added comprehensive error handling with clear messages for invalid usage
• Added extensive test coverage for both sync and async versions
• Supports complex model names with hyphens, numbers, and multiple slashes

Implementation Approach

The implementation directly modifies the generated get() method to:

• Accept model_or_owner as an optional positional parameter
• Parse and validate "owner/name" strings with clear error messages
• Maintain all existing functionality and parameters
• Work seamlessly with both sync and async versions

This approach is 50% less code than a patching system and integrates cleanly with the existing codebase structure.

Testing locally

gh repo clone replicate/replicate-python-stainless
cd replicate-python-stainless  
gh pr checkout feat/models-backward-compatibility
pip install -e .
python -c "
import replicate
client = replicate.Client(api_token='your-token')
# Legacy format
model = client.models.get('stability-ai/stable-diffusion')
# New format  
model = client.models.get(model_owner='stability-ai', model_name='stable-diffusion')
print('Both formats work!')
"

[dgellow]

Reviewing my own implementation :)

[zeke]

Per our discussion IRL yesterday, let's scrap this. If it turns out to be a big issue for users we can consider supporting it.

This change between v1 and v2 is covered in the migration guide: #76