Feat/add exists method

[SaiDevaHarshaR]

What this does

Adds exists() method to check if a model is available locally
without manually catching exceptions or iterating through list().

Usage

import ollama
ollama.exists("llama3.2")   # True
ollama.exists("not-a-model") # False

Why

Fixes #640 - users wanted a clean way to check model availability
without try/except boilerplate in their own code.

How

Calls show() internally and returns True if it succeeds,
False if it raises an exception.