feat: add client.exists() to check if a model is available locally

[CyberRaccoonTeam]

Summary

Adds a convenience method client.exists(model_name) to check if a model is available locally without raising an exception.

Problem

Currently, checking if a model exists requires either calling client.show() and catching an exception, or listing all models and filtering. This is verbose and error-prone.

Solution

# Before
try:
    client.show("llama3.2")
    exists = True
except ResponseError:
    exists = False

# After
exists = client.exists("llama3.2")  # Returns True/False

Changes

• Added exists() method to Client class
• Returns True if model is available locally, False otherwise
• Handles ResponseError internally
• Added tests for the new method

Testing

• ✅ Test with existing model returns True
• ✅ Test with non-existing model returns False
• ✅ All existing tests pass