fix: Remove pydantic constraint <2.10.0 and update timedelta validator, serializer type hints (#757)

Pydantic version 2.10.0 made some changes that broke our code. This was fixed in 2.10.3

Also our type hints were too generic in models.py. This change makes them more specific.
Older versions of Pydantic 2.6.0-2.8.0 caused errors in our codebase and so minimal version of Pydantic was bumped to 2.8.1.

Author: Pijukatel