fix: drop pickled RDS instance from session

verify_credentials previously stored pickle.dumps(rds, -1) under
session['aws']['aws_rds_obj']; subsequent endpoints unpickled it via
pickle.loads. That pickle round-trip was an in-session deserialization
vector and forced the session storage to carry binary blobs.

The RDS class is a thin wrapper over boto3 clients: all of its mutable
state is the credential dict already saved at session['aws']['secret'].
Drop the pickled blob and reconstruct RDS per request via a new
_get_rds_from_session() helper. boto3 client construction is cheap
(microseconds) and clients are cached on the per-request instance.

Also fixes a pre-existing latent bug in verify_credentials where status
was undefined when the cached creds matched the new ones (only the
inside of the cache-miss branch assigned it), now explicitly set to
True for the cache-hit path.

Five new tests cover: full creds, missing session_token, no aws key,
no secret key, and a regression assertion that the cloud.rds module
no longer imports the unsafe deserializer.