sec(storage): implement in-memory secret scrubbing for database passwords and credentials (#607)

[ZhuchkaTriplesix]

Summary

Implements zero-trust memory security by clearing and scrubbing in-memory database passwords and connection URI secret buffers immediately after establishing database connections.

Changes

1. ConnectionRow:
   • Added withoutSecrets() method to return a sanitized copy with password and connectionString scrubbed to null.
   • Added credential status helpers: hasPassword, hasConnectionString, and hasSecrets.
2. Database Drivers:
   • Implemented scrubCredentials() in PostgresConnection, MysqlConnection, RedisConnection, and MongoConnection.
   • Scrubbed sensitive in-memory credentials immediately upon successful handshake / authentication.
   • Added secure on-demand credential re-hydration from ConnectionSecretsStore when reconnecting scrubbed instances.
3. Tests:
   • Added unit tests in test/core/storage/secret_scrubbing_test.dart validating that credentials are scrubbed across connection models.

Closes #607