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

[CyberRaccoonTeam]

Summary

Implements #640 — adds exists() method to Client and AsyncClient that checks if a specific model is already pulled locally.

Changes

• Added Client.exists(model: str) -> bool — wraps list() to check for a model by name
• Added AsyncClient.exists(model: str) -> bool — async version
• Added top-level ollama.exists() convenience function
• 5 tests covering: found, not found, empty list, async variants

Usage

import ollama

# Check before pulling
if not ollama.exists("llama3.1:8b"):
    ollama.pull("llama3.1:8b")

# Or with client
client = ollama.Client()
if client.exists("qwen2.5:latest"):
    response = client.chat("qwen2.5:latest", messages=[...])

Testing

5 tests — all passing
- test_client_exists_true
- test_client_exists_false  
- test_client_exists_empty_list
- test_async_client_exists_true
- test_async_client_exists_false