fix(deps): add missing requests-toolbelt dependency to pyproject.toml

[verseon0980]

Problem

requests-toolbelt is imported at the top level of
src/opengradient/client/model_hub.py:

from requests_toolbelt import MultipartEncoder

This file is imported by client/__init__.py, which is imported by
opengradient/__init__.py. This means the very first line of any
user code:

import opengradient as og

crashes immediately on a clean install with:

ImportError: No module named 'requests_toolbelt'

requests-toolbelt is not listed anywhere in the dependencies
section of pyproject.toml. So pip install opengradient does not
install it. The entire SDK is broken for any user who installs it in
a fresh environment.

Affected files:

• src/opengradient/client/model_hub.py (line 9)
• pyproject.toml (dependencies section)

Fix

Added requests-toolbelt>=0.9.1 to the dependencies list in
pyproject.toml. This ensures it is installed automatically whenever
a user runs pip install opengradient.

No code changes needed. The import in model_hub.py is correct.
Only the dependency declaration was missing.

Changes

• pyproject.toml: added "requests-toolbelt>=0.9.1" to the
  dependencies list under [project]