fix: Add missing requests-toolbelt and langchain-core dependencies

[Kewe63]

Description

This PR resolves critical ModuleNotFoundError issues that occur on fresh installations of the OpenGradient SDK by explicitly adding missing packages to the project dependencies.

Previously, requests-toolbelt and langchain-core were not defined in pyproject.toml, which caused the following components to fail outright:

• model_hub.py: Failed to import MultipartEncoder from requests_toolbelt.
• alphasense and pytest: Failed to locate langchain_core.

Changes Made

• Added requests-toolbelt>=1.0.0 and langchain-core>=0.3.15 to the dependencies list in pyproject.toml.
• Synchronized uv.lock to reflect and lock the newly added dependencies.

Testing

• Successfully ran make test equivalents against the isolated .venv environment (121 passing tests).
• Confirmed mypy static type checking passes indicating imports are resolvable.