This PR contributes to#1508 by fully typing thegoogle/auth/jwt.py module usingmypy --strict.

✅ Highlights:

• Full type annotations added to all public methods and classes
• Proper handling of byte-to-string conversion in error formatting
• Replaced f-strings with.format() to comply with--strict mode
• Explicitstr casts where required to eliminatestr-bytes-safe warnings
• # type: ignore used only where necessary (MalformedError,_helpers)
• Removed deprecated# type: ignore lines and updated Mapping fallback

🧪 Verified With:

mypy --strict google/auth/jwt.py