sec(storage): implement in-memory secret scrubbing for database passwords and credentials

[ZhuchkaTriplesix]

Motivation & Goal

Enhance zero-trust memory security by clearing/scrubbing in-memory password representations and plaintext credential buffers immediately after establishing database connections.

Proposed Scope

• Zero-out or discard sensitive password buffers in ConnectionRow, ConnectionSecretsStore, and driver connection factories once authentication handshake succeeds.
• Avoid retaining plaintext database passwords in long-lived client state models when only connection handles/leases are needed.
• Add unit tests validating that sensitive in-memory credentials are not retained across active connection lifecycles.