feat: add exists() method and fix ShowResponse for cloud models (clos…

[shivanireddyk]

…es #640, closes #607)
Closes #640
Closes #607

Changes

New: ollama.exists(model)

A clean boolean check for whether a model is available locally —
no more catching exceptions for control flow.

# Before (ugly)
try:
    ollama.show("llama3.2")
except ollama.ResponseError:
    ollama.pull("llama3.2")

# After (clean)
if not ollama.exists("llama3.2"):
    ollama.pull("llama3.2")

Available on both Client and AsyncClient.

Bug fix: ShowResponse crash on cloud models (#607)

model_info was declared as required by Pydantic even though it was
typed Optional. Cloud models like glm-4.7:cloud don't return
model_info, causing a hard ValidationError. Fixed by adding
default=None.