fix(connections): import DBeaver username from credentials-config.json (#1355)

[datlechin]

Fixes #1355.

Importing connections from DBeaver left the username empty. The importer read the user from data-sources.json's configuration block, but DBeaver 6.1.3+ stores the username in the encrypted credentials-config.json under #connection.user, not there. The value was decrypted and parsed but never read onto the connection.

Changes

• Read the username from credentials-config.json #connection.user, falling back to data-sources.json configuration.user (used by admin/env-template configs).
• Load the credentials file regardless of the "include passwords" choice. The username is connection metadata, not a secret; password import stays gated on that choice.

Tests

Added 4 tests: username from credentials, username imported when passwords are excluded, fallback to configuration.user, and credentials-wins precedence. All pass.