Fix OIDC token instrospection redis_instance args

[davinotdavid]

Description of the Change

• When we call get_subscriber directly, we are not going through the FastAPI DI so the redis_instance is actually a Depends object. Added the redis_instance to the calls to get_subscriber so that it actually works now.
• Also, since there might be other occasions in the future where we might make the call directly, I've added a conditional to make the redis_instance = None instead so we fail gracefully and just don't use the caching at all for the token introspection (and added tests).

Benefits

• Less noisy Sentry logs and gracefully handling cases where we mistakenly call the get_subscriber without passing a redis_instance from the FastAPI DI.

Applicable Issues

Fixes #1448

[MelissaAutumn]

Looks good, I'd rather we resolve the dependency for redis_instance if it isn't already, but we can cut a future ticket for that.

[devmount]

lgtm, thanks Davi!