build(deps): bump pydantic and openai dependencies

[ajndkr]

Description

Fixes #148 Fixes #141 Fixes #110

Changelog:

• ⬆️ bumped pydantic and openai dependencies
• ♻️ migrated codebase to pydantic_v2
• ✅ fixed breaking unit tests
• ➕ added uvicorn as dev dependency (useful for running test applications)
• 🔥 removed openai_aiosession decorator from StreamingResponse (redundant after openai_v1 migration)
• 🔥 removed pydantic dependency from callback handler classes (langchain doesn't use pyndantic for the native handlers)