This PR contributes to #1508 by fully annotating the following modules to support mypy --strict compatibility:

✅ Files Updated

• google/auth/transport/__init__.py – fully typed abstract base classes for Request and Response
• google/auth/transport/requests.py – concrete implementation with strict typing

📌 Highlights

• All functions and methods include explicit return type annotations
• Deprecated @abc.abstractproperty replaced with @property + @abc.abstractmethod
• __call__() in Request interface now explicitly typed
• All mypy strict errors resolved, including:
  • Line 53: missing return on status
  • Line 58: missing return on headers
  • Line 63: missing return on __call__
  • Line 78: untyped helper function

✅ Verified

mypy --strict google/auth/transport/__init__.py
mypy --strict google/auth/transport/requests.py