fix: log rollback errors and use O(1) string length

[datlechin]

Summary

• DatabaseManager+Schema.swift: try? on rollbackTransaction() now wrapped in do/catch with logger.error so rollback failures are visible instead of silently swallowed
• CompletionTextField.swift: string.count (O(n) in Swift) replaced with (string as NSString).length (O(1))
• EditorEventRouter.swift: Added clarifying comment on why DispatchQueue.main.async inside MainActor.assumeIsolated is intentional (coalesces rapid didUpdateNotification fires)

Test plan

• Build succeeds
• Filter text field autocomplete still works correctly
• SQL editor cursor/first-responder behavior unchanged