fix(mongodb): handle unauthorized listDatabases on admin by falling back to connection database

[ZhuchkaTriplesix]

Summary

In MongoConnection.listDatabases():

final adminDb = await openDatabase('admin');
final result = await adminDb.runCommand({'listDatabases': 1});

On managed MongoDB hosting providers (such as MongoDB Atlas shared tiers M0/M2/M5, DigitalOcean, or standard database-scoped user accounts), users are only granted readWrite privileges on their designated target database (e.g. my_app_prod). They lack the cluster-level listDatabases privilege on the admin database.

When openDatabase('admin') or runCommand({'listDatabases': 1}) is executed, MongoDB returns an unauthorized error (code: 13). In MongoDatabasesView, this error is caught and sets _error = 'Failed to list databases: ', permanently blocking the user from seeing or browsing any collections, even though a valid database was configured in ConnectionRow.database.

Repro

1. Connect with credentials that have read/write access to testdb only, without cluster privileges on admin.
2. Open the MongoDB database explorer.
3. Observe that listDatabases fails with an authorization error, displaying a full-screen error with no way to access testdb.

Scope

• Catch authorization errors in listDatabases() or in MongoDatabasesView._loadDatabases().
• If listing all cluster databases fails and connectionRow.database is provided, fall back to returning [connectionRow.database].
• Allow the user to proceed to browse collections within their accessible database without obstruction.

Out of scope

• Dynamic permission probing.

[ZhuchkaTriplesix]

Resolved by #930 (already merged into dev; not auto-closed since this repo's default branch is main and PRs merge into dev).