fix(types): add full type annotations to oauth utils and exceptions (#1508)

[cureprotocols]

This PR contributes to #1508 by fully typing both google/oauth2/utils.py and google/auth/exceptions.py under --strict mode.

✅ Changes

• Added explicit return and argument types across both files.
• Enhanced OAuthError with optional response_body to allow better downstream debugging.
• mypy --strict verified ✅ on both files.

These changes are isolated, fully backward-compatible, and bring the codebase closer to strict typing compliance.

Let me know if anything else needs to be adjusted!