Python: all: Standardize TypeVar naming convention to use suffix T

[eavanvalkenburg]

The convention should be to suffix TypeVars with T instead of prefix. For example:

• ChatResponseT instead of TChatResponse

File: python/packages/core/agent_framework/_types.py
Location: Throughout, e.g., line ~2198

Action: Standardize TypeVar naming convention across the codebase.

Parent issue: #3575