[FIX] v3.x: Match synced MySQL users by (username, host) to avoid collapsing multi-host accounts

[erhanurgun]

Closes #1074

SyncDatabaseUsers was looking up existing rows by username only, so on a server with both 'app'@'localhost' and 'app'@'127.0.0.1' one row kept being overwritten on every sync and the second one was never inserted. The Vito UI then ended up showing a databases list that didn't match the host above it.

Fix is one extra where('host', ...) when the handler returns a non-empty host. Postgres is left on the username-only path because its get-users-list view returns '' for host (roles are host-agnostic) and an unconditional composite filter would insert a duplicate (rolname, '') row on every sync.

  /** @var ?DatabaseUser $databaseUser */
- $databaseUser = $server->databaseUsers()
-     ->where('username', $user[0])
-     ->first();
+ $query = $server->databaseUsers()->where('username', $user[0]);
+ if ($user[1] !== '') {
+     $query->where('host', $user[1]);
+ }
+ $databaseUser = $query->first();

vendor/bin/phpunit tests/Feature/DatabaseUserTest.php -> 15/15 green; three new tests cover the multi-host case, idempotent re-sync, and the Postgres no-duplicate regression. CreateDatabaseUser, UpdateDatabaseUser, and DeleteDatabaseUser already scope by (username, host), so this just brings sync in line.

No migration. A composite unique index on (server_id, username, host) would be a natural follow-up but felt out of scope.

[saeedvaziry]

@erhanurgun I appreceate the efforts on the issues and PRs however I rather to skip the long reads with AI generated too much details.

Lets keep the issues and PR descriptions for us humans with our own words.

Thanks

[erhanurgun]

Sorry for the wall of text, you're right. Trimmed both the PR description and the issue.